Science of Sintering 50 (2018) 95-109
________________________________________________________________________
_____________________________
) Corresponding author miljanamvincars
doi httpsdoiorg102298SOS1801095M
UDK 661143 622785
Structural Morphological and Electrical Properties of Multi-
Doped Calcium Phosphate Materials as Solid Electrolytes for
Intermediate Temperature Solid Oxide Fuel Cells
Miljana Mirković1)
Anja Dosen1 Suzana Erić
2 Marija Stojmenović
1
Branko Matović1 Aleksandra Rosić
2
1Institute of Nuclear Sciences Vinča University of Belgrade Mihaila Petrovića
Alasa 12ndash14 11001 Belgrade Serbia 2Faculty of Mining and Geology University of Belgrade Đušina 7 11001 Belgrade
Serbia
Abstract
Modified solution precipitation method was used to prepare pure and doped Mg Sr
and Na hydroxyapatite type materials (CaP CaMgP and CaSrNaP) Modification consisted
of partial substitution of nitrates by acetate solution in order to achieve a more soluble and
cost effective synthesis The obtained samples were calcined at 400 ordmC (CaP400 CaMgP400 and
CaSrNaP400) All powders were characterized by X-ray diffraction (XRD) Fourier transform
infrared (FTIR) spectroscopy and scanning electron microscopy (SEM) Calcined samples
were densified at 1000 ordmC in an air for 3 h (CaP1000 CaMgP1000 and CaSrNaP1000) Sintered
samples were characterized by XRPD FTIR SEM EDS and complex impedance methods
The highest conductivity was found for the multi-doped phosphate sample (CaSrNaP1000) at
700 ordmC (190times10-3
Ω-1
cm-1
) The corresponding activation energies of conductivity amounted
to 031 eV in the temperature range 500-700 ordmC
Keywords Sintering Xndashray diffraction Electrical conductivity Phosphate materials Fuel
cells
1 Introduction
Due to the high efficiency and applicability of different types of fuels (biogas natural
gas hydrogen and methane) [1-4] Intermediate Temperature Solid Oxide Fuel Cells (ITndash
SOFCs) have become one of the most promising energy conversion devices and emphasis is
placed on the development of microndashSOFCs as power sources [5] which could operate at
reduced temperatures in porTab electronic devices To achieve this goal it is necessary to
synthesize material with corresponding properties Generally the conductivity of these
systems depends on the processing and intrinsic properties of the material itself where micro
structural features and density have a key role [6] Recently cerium (IV) oxide (CeO2) solid
solution materials became favored for potential use in ITndashSOFCs with appropriate doping
methods may create fluoritendashtype crystal lattice which has a good influence at ionic
conductivity but the use of CeO2 and oxide rarendashearth is very expensive [7-10] An adequate
substitute for CeO2 might be calcium phosphate hydroxyapatite HA ceramics - Ca5(PO4)3OH
due to their physical and chemical properties and very low cost of synthesis [11 12] Various
types of synthesis were used for preparation HA ceramics solndashgel processes [13]
M Mirković et al Science of Sintering 50 (2018) 95-109
___________________________________________________________________________
96
hydrothermal synthesis [14] and solid state reaction method [15] In comparison to these
methods direct precipitation from aqueous solution provides an easier way of preparing HA
ceramic and at the same time provides a large amount high purity material [16-18]
Furthermore ion substituted calcium phosphate materials can be prepared using
simple chemical precipitation method which is of great importance when it comes to
obtaining a solid ionic conductor for use in ITndashSOFCs the advantages of this method include
simple synthesis low cost and ability to obtain large quantity and high purity of solid
electrolyte [18]
In recent years ionic substitutions of Mg2+
Na+ and Sr
2+ in HA [11 12] proved very
interesting for their potential implementation in ITndashSOFCs owing to the morphology
thermal stability and structural and mechanical properties [19] Lowndashtemperature treatment
(400ndash500 ordmC) allows preserving the particle morphology and developing nanoscale structures
ie grains [20 21] In order to obtain compact material densification process usually requires
sintering at temperatures at or above 1000 ordmC however this leads to the formation of high
temperature calcium phosphate ceramics [22-24] Knowledge of thermal stability ionic
transport properties and possible mechanisms of conduction properties of different calcium
phosphate materials provides a key for their application in ITndashSOFCs [11 12] In this study
we show Mg2+
Na+ and Sr
2+ HA materials as suiTab candidates for ionic conductive
materials Materials were treated thermally in air atmosphere at 400 ordmC (CaP400 MgCaP400 and
NaSrCaP400) Additionally behavior doped CaP after sintering at 1000 ordmC during 3 h (CaP1000
MgCaP1000 and NaSrCaP1000) was studied The main objective was to obtain a range of dense
solid solution calcium phosphate materials with suiTab morphology and grain size for the
application in Intermediate Temperature Solid Oxide Fuel Cells
2 Material and methods
21 Synthesis of CaP CaMgP and CaSrNaP samples
Samples were prepared by solution precipitation method Pure hydroxyapatite (CaP)
was prepared by drop wise addition of 250 ml 03 M NaH2PO4+H2O into 250 ml Ca(OH)2
05 M solution which was magnetically stirred The pH was adjusted to 11 using NH4OH at
temperature about 100 ordmC To synthesize Mg (CaMgP) and NaSr (NaSrCaP) substituted
hydroxyapatite materials we used slightly modified precipitation method To obtain MgHA
250 ml NaH2PO4+H2O 03 M was heated at 100 ordmC and added drop wise into mechanically
stirred mixture of 250 ml Ca(OH)2 05 M and 250 ml MgCl2∙6H2O 025 M heated to about
100 ordmC pH was adjusted to 12 using NH4OH solution For NaSrCaP synthesis mixture of 250
ml of (CH3COOH)2Ca∙H2O 003 M and 250 ml Sr(CH3COO)2 003 M was added drop wise
into 500 ml NaH2PO4+H2O 03 M at room temperature The precipitates were washed
centrifuged at 3000 rpm and dried at 70 ordmC for 17 h Dry powders were thermally treated at
400 ordmC during 3h in air atmosphere and then uniaxially pressed at 105 MPa into compact
pelets (8 mm diameter) and sintered at 1000 ordmC for 3 h in air atmosphere
22 Densification of CaP CaMgP and CaSrNaP samples
Samples were uniaxially pressed at 105 MPa and thermally treated Sample annealing
was carried out at 1000 ordmC for 3 h in an air at a heating rate of 5 ordmCmin The resulting density
was determined via Archimedesrsquo method Densified samples were characterized using X-ray
diffraction (XRD) scanning electron microscopy (SEM) Energy Dispersive Spectroscopy
(EDS) and complex impedance methods
M Mirković et alScience of Sintering 50 (2018) 95-109 ___________________________________________________________________________
97
23 Characterization of CaP CaMgP and CaSrNaP samples
Phase composition before and after thermal treatment as well as crystallite size
evolution were analyzed by X-ray diffraction (XRD) using Ultima IV Rigaku diffractometer
equipped with CuKα radiation using a generator voltage 400 kV and a generator current 400
mA The range of 5ndash80deg 2θ was used in a continuous scan mode with a scanning step size of
002deg and count time 2degmin Analysis was done using PDXL2 software (version 2030)
[25] with reference to ICDD database [26] For quantitative phase analysis RIR method was
used Structural modeling was done using VESTA program [27] Spectroscopic studies of the
synthesized and thermally treated materials were carried out in the mid infrared (MIR)
regions (4000ndash400 cm-1
) using Fourier transforms infrared (FTIR) spectroscopy in
transmission mode by a Perkin Elmer Spectrum Two FT-IR spectrometer using the pressed
KBr compacts (1100) technique
The morphology and chemical composition of samples were identified using a JEOL
JSM-6610LV Scanning Electron Microscope with an X-Max Energy Dispersive
Spectrometer Samples were coated with gold using a BALTEC-SCD-005 sputter coating
device and the results were recorded under high vacuum conditions The electrical
conductivity of sintered samples was measured by complex impedance method in a
frequency range 1 Hz-01 MHz using Interface 1000 Potentiostat Galvanostat ZRA and
EIS300 Electrochemical Impedance Spectroscopy Software The measurements were
conducted in air in the temperature range 500-700 ordmC with a 50 ordmC increment The amplitude
of the applied sinusoidal voltage signal was 20 mV Thin layer of high conductivity silver
paste was applied onto both sides of the sample pellets in order to provide good electrical
contact between electrolyte and electrodes The samples were placed between the silver plates
in a ceramic holder which was heated by vertical oven A PtndashRh thermocouple located just
below the bottom silver plate was used for temperature monitoring The impedance plots
obtained experimentally were fitted by means of the software ZViews for Windows (Version
32b) The resistance values were determined from the impedance diagrams recorded at
various temperatures The specific conductance was calculated from the resistance data using
the dimensions of the sample pellets
3 Results and discussion
31 Structural and morphological properties of CaP CaMgP and CaSrNaP
samples
311 XRD analysis
X-Ray diffraction patterns of synthesized pure hydroxyapatite (CaP) powders
calcined at 400 ordmC (CaP400) and sintered at 1000 ordmC (CaP1000) during 3 h are shown in Fig 1
CaP powder shows broad peaks typical for hydroxyapatite obtained by precipitation method
CaP and CaP400 crystallized as hexagonal Ca5(PO4)3OH sg P63m (ICDD 01-071-5048)
XRD results show the formation of another calcium phosphate phase in the sintered CaP1000
sample Based on peak identification structural refinement and decomposition calculation
using PDXL2 software we found that CaP1000 consists of 85 β - Ca2P2O7 (ICDD 01-071-
2123) phase and of 15 hydroxylapatite phase (ICDD 01-071-5048)
XRD diffraction patterns of Mg doped HA are shown in Fig 2 Analyses show almost
identical patterns as pure HA powder however peaks are shifted compared to pure HA
material (Fig 1) suggesting that Mg2+
cations were incorporated in the structure during
thermal treatment at 400degC XRD of CaMgP1000 sample shows that heating to 1000 ordmC leads to
the formation of high temperature magnesium phosphate phases Mg2P2O7 (ICDD 00-008-
0038 marked as open circles) and β-Ca2P2O7 (ICDD 01-071-2123 marked as solid circles)
M Mirković et al Science of Sintering 50 (2018) 95-109
___________________________________________________________________________
98
It is evident that addition of magnesium to the HA structure leads to the formation of high
temperature solid solution that includes high temperature CaMgP phases and hydroxyapatite
CaMgP100 nominal phase composition calculated using PDXL2 software showed 55 β-
Ca2P2O7 30 Mg2P2O7 and 15 hydroxyapatite phase
Fig 1 XRD patterns of CaPndashhydroxyapatite synthesized sample CaP400ndashhydroxyapatite
calcinated at 400 ordmC for 3 h in air and CaP1000ndashsintered at 1000 ordmC for 3 h in air
Fig 2 XRD patterns of CaMgPndashsynthesized sample CaMgP400ndashcalcined at 400 ordmC for 3 h in
air and CaMgP1000ndashsintered at 1000 ordmC for 3 h in air
The diffraction pattern of powder CaSrNaP (Fig 3) reveals that the dried precipitate exhibits
poor crystallinity peaks around 10ordm and 18deg 2θ (marked as inverted triangles) indicate
secondary phase with chemical composition Sr3(P3O9)2(H2O)7 (ICDD01-070-0007) The
secondary phase existence is common during precipitation method of nano material synthesis
[28] However there is no evidence of the existence of secondary phase in thermally treated
CaSrNaP400 (Fig 3) CaSrNaP XRD peaks are slightly shifted to the left implying a larger unit
cell most likely due to the incorporation of Sr into the HA lattice (Fig 3) Thermal treatment
of hydroxylapatite materials below 700 ordmC leads to surface reduction and single phase particle
coalescence without densification [24 29] At 1000 degC high temperature solid solution forms
noted as CaSrNaP1000 Sintering at this temperature with retention time of 3 h high
M Mirković et alScience of Sintering 50 (2018) 95-109 ___________________________________________________________________________
99
temperature calcium and strontium and sodium phosphate materials developed It is evident
that peaks belongs to three different phosphate phases β ndash Ca2P2O7 (01-071-2123) α-Sr2P2O7
(ICDD 01-072-149) and Na4P2O7 (ICDD 01-073-5982) Strontium and sodium phosphate
phases are orthorhombic while the calcium phosphate phase is tetragonal Structural changes
and phase transitions during sintering (temperatures above 700 degC) can be explained by the
loss of hydroxyl ions from the hydroxyapatite structure at temperatures around and higher
than 1000 degC and the formation of orthorhombic and tetragonal phosphate phases [24 30]
The peaks on XRD diagram are narrow and well defined indicating high structural
arrangement of sintered material
Quantitative phase analysis of CaSrNaP material showed 74 of α-Sr2P2O7 16 of β
ndash Ca2P2O7 and 10 of Na4P2O7 Hydroxylapatite is still evident at high temperatures in the
case of pure HA and HA doped with Mg On the other hand in the case high temperature
treatment (1000 degC for 3h) of multi-doped HA leads to the formation of mixed CaSrNaP
phase with high structural arrangement
Fig 3 XRD patterns of CaSrNaPndashsynthesized sample CaSrNaP400ndashcalcined at 400 ordmC for 3 h
in air and CaSrNaP1000ndashsintered at 1000 ordmC for 3 h in air
Tab I Unit cell parameters and crystallite sizes of obtained materials
Samples Cell parameter (Aring)
Crystallite size (nm) a c
CaP 938(14) 692(19) 14(13)
CaP400 939(10) 691(12) 152(5)
CaMgP 940(4) 687(5) 72(10)
CaMgP400 935(3) 688(3) 156(9)
CaSrNaP 934(4) 679(4) 43(5)
CaSrNaP400 945(6) 729(7) 79(14)
M Mirković et al Science of Sintering 50 (2018) 95-109
___________________________________________________________________________
100
Tab I shows variation of lattice parameters due to structural substitution during
precipitation and calcinations at 400 degC Unit cell parameters and crystallite size calculations
of were performed using cif files from American Mineralogist Crystal Data Structure Base
(AMCDSB)[31] (Tab I)
There is no significant difference in the average crystallite size of CaP and CaP400
samples (Tab I) Unit cell parameters of both samples are in similar to pure hydroxyapatite
(ICDD 01-071-5048) With the increase of temperature unit cell parameter a increases while
the parameter c decreases Average crystallite size of CaMgP material increasing with
temperature which is consistent with peak shapes of obtained material The unit cell
parameters of CaMgP are higher than of CaP material which is in correlation with published
data [32] Average crystallite size of thermally treated samples is slightly higher than
synthesized Factors that influence peak broadening are the presence of amorphous phase
during precipitation and a number of dislocations between crystallite boundaries [16 33]
During calcination at 400 ordmC the material becomes crystalline and thermodynamically sTab
allowing for better arrangement and growth of crystallites [34] The unit cell parameters of
CaSrNaP and CaSrNaP1000 have the highest value out of all examined materials as the ionic
radii of Sr2+
(113 Aring) and Na+ (152 Aring) are larger than ionic radii Ca
2+ (099 Aring) and Mg
2+
(069 Aring) Anisotropic change in unit cell dimensions could be attributed to the replacement of
Ca2+
with other cations ie Mg2+
Na+ and Sr
2+ in structure [35 36]
312 FTIR analysis
FTIR spectra of untreated (synthesized) samples CaP CaMgP and CaSrNaP (Fig 4)
showed the presence of characteristic apatite signals (around 563 610 657 1030 1077 cm-1
)
Fig 4b [36] The absorbed water band is around 3500 cm-1
and the band at 1413 corresponds
to CO32-
group vibration [18] In comparison with FTIR of stoichiometric HA bands of
CaSrNaP and CaMgP are red-shifted because of the presence of foreign ions incorporated into
the structure [37-39] Absence of the 631 cm-1
band that belongs to vibration modes of the
apatitic OH groups in all tree spectra most likely indicates a lack of hydroxyl content and a
possible substitution of hydroxyl groups by carbonate groups [37]
Fig 4 FTIR spectra of untreated (synthesized) samples a) CaMgP b) CaP and c) CaSrNaP
M Mirković et alScience of Sintering 50 (2018) 95-109 ___________________________________________________________________________
101
Furthermore the FTIR spectrum of CaMgP shows a slight indication of 1410 cm-1
carbonate band (Fig 4a) thus indicating no significant presence of CO3 group in CaMgP
powder [40] Weak band of MgHCO3 can be noticed around 1634 cm-1
[36] CaSrNaP band at
1385 cm-1
indicates incorporation of Sr2+
and Na+ ions into the structure of hydroxylapatite
(Fig 4c) [35 38] A band that indicates carbonate group (coming from the reaction medium)
in the structure of CaSrNaP is demonstrated by bands at 1417 and 1639 cm-1
[41] FTIR
spectra confirm previously described XRD results that using acetate solution multi doped
hydroxyapatite material can be obtained
FTIR spectra of thermally treated samples at 1000 ordmC for 3 h in air are shown in Fig
5 also shows presence of the characteristic signals of apatite [36] CaMgP1000 (Fig 5a)
sample shows bands in spectral regions 1470-870 cm-1
and 605-560 cm-1
which are generally
related to P-O species with different phosphorous configuration [42 43] Characteristic
vibrations of Mg2P2O7 phase are bands at 1050 605 and 560 cm-1
[42] The bands at around
1050 cm-1
belong to triply degenerate asymmetric P-O stretching mode while the band
around 955 cm-1
belongs to P-O stretching mode these bands belong to β-Ca2P2O7 phase [43]
There is no evidence of any characteristic hydroxyl group vibrations (631 and 3572 cm-1
)
CaP1000 (Fig 5b) bands are weaker than in untreated samples indicating lack of water
vibrations in structure The bands around 600 and 580 cm-1
are attributed to different types of
P-O-P bending mode vibrations which belong to β-Ca2P2O7 [43] Figure 5c shows vibration
spectrum of CaSrNaP1000 material Presence of previously described P-O and P-O-P vibrations
which belong to β-Ca2P2O7 phase also belong to Sr2P2O7 [43 44] The band around 740 cm-1
belongs to P-O-P bridge stretching vibration a striking feature of (P2O7)4-
ions in Sr2P2O7
FTIR spectrum of CaSrNaP (Fig 5c) shows no evidence of O-H stretching modes at around
3500 and 1600 cm-1
[44] Metal-oxygen vibrations in these types of systems are generally too
low to be seen in the 400-4000 cm-1
region
Fig 5 FTIR specta of thermally treated samples at 1000 degC in air for 3 h a) CaMgP1000 b)
CaP1000 and c) CaSrNaP1000
M Mirković et al Science of Sintering 50 (2018) 95-109
___________________________________________________________________________
102
Based on FTIR analysis at 1000 degC high temperature CaMgP1000 material is obtained
All of identified vibrations that belong to CaP1000 material (Fig 5b) confirm that β-Ca2P2O7 is
obtained by sintering hydroxyapatite for 3h on 1000degC in air atmosphere The analysis of
CaSrNaP1000 spectrum (Fig 5c) confirms absence of O-H vibrations and presence of (P2O7)4-
vibrations which is unique for X2P2O7 types of compounds [43 44]
313 Morphology (SEM analysis) and element composition (EDS analysis)
SEM analysis confirms that all obtained samples using precipitation method have
crystallite size of nanometer dimensions In addition SEM images (Fig 6) show that the
particle size in all samples increases with the increase of temperature Namely nanoparticles
have a natural tendency to agglomerate for two reasons firstlyminusthe agglomeration is a more
sTab configuration from an energetic point of view and secondlyminusthe agglomeration allows
the crystallite growth [45 46]
Fig 6 SEM micrographs of A) CaP CaP400 and CaP1000 B) CaMgP CaMgP400 and
CaMgP1000 C) CaSrNaP CaSrNaP400 and CaSrNaP1000 with EDS spectra
Sintering at 1000 ordmC in air for 3 h led to a higher degree of densification of
CaSrNaP1000 (98 TD) than of CaMgP1000 (86 TD) The main reason for the differences in
density may be pore and particle size distribution ie different morphology (Fig 6) The
samples after calcinations at 400 ordmC during 3 h show more uniform particles and well ordered
structure Calcinations was done in order to allow better packing and lower inner activity and
it was expected that these samples may achieve higher density after sintering This was
achieved for CaSrNaP1000 sample where the grains are uniformly sintered and the contacts
between grains show hexagonal forms and the inserted section clearly demonstrates that we
have indeed obtained dense high temperature CaSrNaP material (Fig 6c) On the other hand
CaMgP1000 (Fig 6b) exhibits grain growth due to formation of agglomerated grains
Densification retards or inhibits wide pore distribution during densification the big pores
M Mirković et alScience of Sintering 50 (2018) 95-109 ___________________________________________________________________________
103
became larger while small pores shrink and disappear [47 48] SEM micrograph of
CaMgP1000 (Fig 6b) shows numerous agglomerates of hexagonal particles with plate like
morphology bonded together with average grain size between 2 and 5 microm with non-uniform
particles with many vacancies less compact particle packing higher inner activity To
confirm the chemical composition the EDS microanalysis was done on the CaP1000
CaMgP1000 and CaSrNaP1000 sintered samples (Fig 6) It revealed that chemical composition
very similar to nominal chemical composition According to semi quantitative chemical
analysis the mean values for the ratios CaPO for CaP1000 sample determined from four runs
are 21275 These ratios correspond to empirically calculated compound with nominal
composition-(Ca21P2O75) The mean value of the chemical ratios CaMgPO for CaMgP1000
was also determined from four runs and amounts to 120727 which corresponds to
empirical compound with following composition-(Ca12Mg07)19P2O7 The mean value of the
chemical ratios Ca Sr Na PO of the NaSrHA1000 sample was determined from four runs
and amounts to 0506227 which corresponds to-Ca05Sr06Na2P2O7 These results (Fig 6)
confirm the assumption that sintered samples based on hydroxyapatite at high temperature
(1000 ordmC) remain sTab and almost nominal chemical composition The obtained EDS results
are in good agreement with XRD results
32 Electrical conductivity
Recently ionic conductors with the apatite type structures attracted a great deal of
attention as an alternative electrolytes to the conventional yttria-stabilized zirconia to operate
in ITndashSOFCs in the intermediate temperature range (600-800 ordmC) [49-52] Namely
hydroxyapatite belongs to apatitic group of phosphate minerals with general formula
Me10(XO4)6Y2 with ionic substitution ability [53] Based on XRD (Tab I) unit cell
parameters of CaMgP samples are slightly increased compared to pure CaP sample the
contraction of a and c parameter can be attributed to much smaller ionic radius of Mg2+
than
Ca2+
[54] The change of unit cell parameters a and c of CaSrNaP indicate that Sr2+
and Na+
ions are incorporated in structure and not just adhered to the crystal where Sr2+
will firstly
replace Ca2+
ion in Ca2 site [55] On the other hand Na+ ion as a smaller than Ca
2+ could
possible replace Ca2+
in Ca1 site while a vacancy may formed at the OH- site [56] At 1000degC
structural transformation of hydroxyapatite occurs and it transforms to a high temperature
mixed phosphate ceramics with M2X2O7 structure where X ions are tetrahedral coordinated
(Fig 7)
Fig 7 Structures of M2X2O7 a) αndashSr2P2O7 b) βndashCa2P2O7 and c) Na4P2O7 (the green spheres
are Sr atoms blue spheres are Ca atoms the yellow spheres are Na atoms the red spheres are
O atoms and the purple spheres are P atoms)
The crystal structures of calcium strontium and sodium phosphate materials are
comprised of diphosphate ions which are linked to metallic ions to form a three
M Mirković et al Science of Sintering 50 (2018) 95-109
___________________________________________________________________________
104
dimensional network in β ndash Ca2P2O7 α-Sr2P2O7 and Na4P2O7 structures [57 58] A
common characteristic of thermal behavior of all phosphates is that under thermal
treatment they undergo dehydratation-condensation reaction which leads to phosphate
tetrahedral bonded to mutual oxygen atom The ion conductivity in these materials
occurs mainly via O- interstitials with preferential cndashaxis conduction [50 51 56 59]
in comparison to O2-
vacancy migration in fluorite and perovskitendashbased electrolytes
[59-63] Generally the impedance spectra are presented as negative of imaginary component
of impedance (ndashZimag) versus real component of impedance (Zreal) ie as Nyquist plot The
semicircles at high and intermediate frequency are ascribed to bulk and grain boundary
processes respectively while semicircle at low frequency represents the electrode process
contribution [56] In our case for the potential application in IT-SOFCs the measurements of
ionic conductivity of electrolytes in solid state of the CaMgP1000 and CaSrNaP1000 samples
were done in intermediate temperature range of 500-700 ordmC with the increments of 50 ordmC
The original Nyquist plots recorded in the available frequency range (1 Hz-100 kHz) are
presented in Fig 8 As it could be seen (Fig 8a and 8b) with increasing the temperature the
values of both resistance elements (Rb and Rgb) obviously decrease which causes an increase
in ωmax Consequently the whole region of the impedance points shifts towards the lowndash
frequency semicircle and at higher temperatures instead of Rb and Rgb separately only the
whole sum Rb+Rgb became readable in the available frequency range In this case at higher
temperatures the time constants associated with the bulk and grain boundary impedances are
much lower than those associated with the electrode interface As a result semicircles due to
bulk and grain boundary disappear and only a single semicircle due to electrode interfacial
processes can be observed [64 66] Therefore only the whole sum Rb+Rig became readable
and the values of total resistance were estimated from the experimental cross section of
obtained semicircles with the real component of impedance (Zreal) this intercept is marked by
arrows in Fig 8 (inset) New semicircles are observed to appear in a lowndashfrequency region
being particularly visible in temperature range at 650-700 ordmC (Fig 8b) Almost doubtless it
originates from the oxygen electrode reactions O2O2-
[64] which does not belong to the
scope of this study
20 30 40 50 60 700
10
20
30
40
50
60
70 a)
20 30 40 50 60 700
1
2
3
4
5
6
7
8
9
10x103
x103
-Zim
ag
(c
m)
Zreal (cm)
x102
x102
500 C
550 C
600 C
-Zim
ag
(c
m)
Zreal (cm)
6 8 10 12 14 160
2
4
6
8
10
12
14
16 b)x102
6 8 10 12 14 160
2
x102
x102
-Zim
ag
(c
m)
Zreal (cm)
x102
650 C
700 C
-Zim
ag
(c
m)
Zreal (cm)
Fig 8 (a)(b) Complex impedance plots of the sintered samples CaSrNaP1000 measured in the
temperature range from 500 to 700 ordmC in air atmosphere The arrows indicate the points on
the real axis corresponding to the readings Rb+Rgb
The total ionic conductivity of the sintered CaSrNaP1000 sample is shown in Tab II Obtained
values (at 700 ordmC amount to 418times10-3
Ω-1
cm-1
) are slightly higher than previously published
ones [64 66] More specifically values of ionic conductivity observed at 700 ordmC in this work
M Mirković et alScience of Sintering 50 (2018) 95-109 ___________________________________________________________________________
105
are similar to values obtained at 800 and 900 ordmC [67-71] This holds also if we compare the
results we found at lower temperatures with the literature data [64-67]
Tab II The temperature dependence of total ionic conductivity (κ) of the sintered CaMgP1000
and CaSrNaP1000 sample
COMPOSITION
κ
(Ω-1
cm-1
)
500 ordmC
κ
(Ω-1
cm-1
)
550 ordmC
κ
(Ω-1
cm-1
)
600 ordmC
κ
(Ω-1
cm-1
)
650 ordmC
κ
(Ω-1
cm-1
)
700 ordmC
CaMgP1000 123times10-4
222times10-4
358times10-4
294times10-4
312times10-4
CaSrNaP1000 227times10-4
356times10-4
657times10-4
117times10-3
190times10-3
According to the results listed in Tab II the dependence log κ = f(1T) of the sintered
sample CaSrNaP1000 is presented in Fig 9 The activation energy of conductivity (Ea) was
calculated from the Arrhenius plot according to the equation
Tk
EAT a 1
lnln (1)
where σ is the conductivity T is the absolute temperature A is the pre-exponential factor and
k is the Boltzmann constant Activation energy of total conductivity for sintered CaSrNaP1000
sample is 031 eV Ea values presented in our work are much lower than previously found [67
68 70 71] In addition comparing the obtained values of Ea with activation energies of
similar oxygen ion conductors containing dopants [71] obtained Ea values were significantly
lower We suggest that this is a consequence of wellndashordered structure and better process
ability of nanopowders obtained application method
100 105 110 115 120 125 130-38
-36
-34
-32
-30
-28
-26
Ea = 031 eV
log
k
1000T (K-1)
Fig 9 The dependence log κ = f(1T) of the sintered CaSrNaP1000 sample
M Mirković et al Science of Sintering 50 (2018) 95-109
___________________________________________________________________________
106
4 Conclusions
The pure Ca and doped with Mg and NaSr hydroxyapatite (CaP CaMgP and NaSrP)
were successfully synthesized using slightly modified solution precipitation method (partial
substitution nitrates by acetate solutions) and thus we achieved higher solubility and more
cost effective synthesis Any secondary phase that formed during synthesis decomposed
during thermal treatment leaving pure CaP CaMgP and CaSrNaP hexagonal (P63m)
Ca5(PO4)3OH materials at room temperature During sintering process at 1000 ordmC in an air for
3 h we developed high temperature doped calcium phosphate materials ie CaMgP1000 and
CaSrNaP1000 materials SEM analysis showed that CaMgP1000 and CaSrNaP1000 materials have
high structural arrangement EDS analysis confirmed that we obtained phosphate compound
type-M2X2O7 The fine crystalline powders after sintering at 1000 ordmC achieved 98 of
theoretical density The maximum value of electrical conductivity at 700 ordmC was achieved for
CaSrNaP1000 material with amounts to 190times10-3
Ω-1
cm-1
Electrical characteristics of the
sintered samples indicate the superiority of solid solution for the application in SOFCs
Additionally considering high sintering temperature one can conclude that the obtained
phosphate material demonstrates high thermal stability an important characteristics in its
potential use as solid electrolyte However it is important to note that the future research
should include furthering lower temperature synthesis routes improving stability ITndashSOFCs
electrolyteelectrode molecular dynamics (MD) simulations of conduction mechanisms and
the general search for improved ion conductors Also the breadth of possible doping regimes
in this phosphate material provides new opportunities to design and optimize their transport
parameters for potential clean energy applications
Acknowledgements
This work was supported by the Ministry of Education Science and Technological
development of Serbia as part of Projects III 45012 and OI 176016
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Bogdanović Mat Chem Phys 153 (2015) 422
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107
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28 (1988) 660
13 F Bakan O Laccedilin H Sarac Powder Technol 233 (2013) 295
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16 I Mobasherpour MS Heshajin A Kazemzadeh M Zakeri J Alloys Comp 430
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19 S C Cox P Jamshidi L M Grover K K Mallick Mat Sci Eng C-Bio S 35
(2014) 106
20 E Champion Acta Biomater 9 (2013) 5855
21 S J Kalita A Bhardwaj H A Bhatt Mat Sci Eng C-Bio S 27 (2007) 441
22 S Ramesh K L Aw R Tolouei M Amiriyan C Y Tan M Hamdi Ceram Int
39 (2013) 111
23 Đ Veljović G Vuković I Steins E Palcevskis P S Usković R Petrović Sci
Sinter 45 (2013) 233
24 S Raynaud E Champion D Bernache-Assollant Biomater 23 (2002) 1073
25 Rigaku Corporation Tokyo J PDXL Version 2030 Integrated X-ray Powder
Diffraction Software 2011
26 Powder Diffraction File P-D announcement of new data-base release 2012
International Centrefor Diffraction Data (ICDD)
27 K Momma F Izumi J Appl Crystallogr 44 (2011) 1272
28 M Prekajski M Mirković B Todorović A Matković M Marinović-Cincović J
Luković B Matović J Eur CeramSoc 36 (2016) 1293
29 J Wang LL Shaw Adv Mater 19 (2007) 2364
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32 G Qi S Zhang K A Khor C Liu X Zeng W Weng Thin Solid Films 516
(2008) 5176
33 S C Cox P Jamshidi L M Grover K K Mallick Mat Sci Eng C-Bio S 35
(2014) 106
34 A Cuccu S Montinaro R Orrugrave G Cao D Bellucci A Sola Ceram Int 41
(2015)725
35 W Zhang Y Shen H Pan K Lin X Liu B W Darvell Acta Biomater 7 (2011)
800
36 J A Gadsden Infrared spectra of minerals and related inorganic compounds
Butterworths London 1975
37 M Iafisco A Ruffini A Adamiano S Sprio A Tampieri Mat Sci Eng C-Bio S
35 (2014) 212
38 S Koutsopoulos J Biomed Mater Res 62 (2002) 600
39 S-C Liou S-Y Chen H-Y Lee J-S Bow Biomaterials 25 (2004) 189
40 M E Fleet Biomaterials 30 (2009) 1473
41 N V Chukanov Infrared spectra of mineral species Extended library Springer
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42 V Koleva V Stefov M Najdoski A Cahil Thermochim Acta 619 (2015) 20
43 B Mehdikhani G H Borhani J Ceram Process Res 16 (2015) 308
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45 G B Balazs R Glass Solid State Ionics 76 (1995) 155
46 G Buvaneswari K Valsalan Mater Lett 134 (2014) 252
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Int40 (2014) 6021
48 A G Evans C H Hsueh J Am Ceram Soc 69 (1986) 444
49 A J Jacobson Chem Mater 22 (2010) 660
50 A Orera P R Slater Chem Mater 22 (2010) 675
51 L Malavasi C A J Fisher M S Islam Chem Soc Review 39 (2010) 4370
52 D Marrero-Loacutepez M C Martiacuten-Sedentildeo J Pentildea-Martiacutenez J C Ruiz-Morales P
Nuacutentildeez M A G Aranda J Power Sources 195 (2010) 2496
53 M P Moreira G D de Almeida Soares J Dentzer K Anselme L Aacute de Sena A
Kuznetsov Mat Sci Eng C-Bio S 61 (2016) 736
54 F Ren Y Leng R Xin X Ge Acta Biomater 6 (2010) 2787
55 Z Y Li W M Lam C Yang B Xu G X Ni S A Abbah Biomaterials 28
(2007)1452
56 Z Leilei L Hejun L Kezhi F Qiangang Z Yulei L Shoujie Ceram Int 40
(2014) 13123
57 K Y Leung C Calvo Can J Chem 50 (1972) 2519
58 H Larsove J Ingela M Christina Acta Chemica Scandinavica 22 (1968) 1419
59 E Kendrick M S Islam P R Slater J Mater Chem 17 (2007) 3104
60 E Kendrick M S Islam P R Slater Chem Commun 715 (2008) 7
61 K Fukuda T Asaka R Hamaguchi T Suzuki H Oka A Berghout Chem Mater
23 (2011) 5474
62 E Kendrick M S Islam P R Slater Solid State Ionics 177 (2007) 3411
63 L Leon-Reina E R Losilla M Martinez-Lara S Bruque M A G J Mater
Chem 14 (2004) 1142
64 N Nallamuthu I Prakash N Satyanarayana M Venkateswarlu J Alloys and
Compounds 509 (2011) 1138
65 M Sato Y Kono H Ueda K Uematsu K Toda Solid State Ionics 83 (1996)249
66 D Marrero-Loacutepez L dos Santos-Goacutemez L Leoacuten-Reina J Canales-Vaacutezquez E R
Losilla Journal of Power Sources 245 (2014) 107
67 S Nakayama T Kageyama H Aono Y Sadaoka J Mater Chem 5 (1995) 1801
68 Y Masubuchi M Higuchi T Takeda S Kikkawa J Alloys and Compounds 408
(2006) 641
69 J McFarlane S Barth M Swaffer J E H Sansom P R Slater Ionics 8 (2002)
149
70 S Tao J T S Irvine Mater ResBullet 36 (2001) 1245
71 J E H Sansom J R Tolchard P R Slater M S Islam Solid State Ionics 167
(2004) 17
Садржај Метода таложне титрације коришћена је за добијање чистог и допираног
хидроксиапатита јонима Sr Mg и Na (CaP CaMgP and CaSrNaP) За синтезу
супституисаних хидроксиапатитских материјала коришћена је модификована
таложна метода у односу на досада познате методе Због боље растворљивости и
знатно веће исплативости коришћени су раствори ацетата уместо нитрата
Синтетисани материјали су калцинисани на 400 degC а затим окарактерисани
рендгенском дифракцијом инфра црвеном спектроскопијом са Фуријеовом
трансформацијом и скенирајућом електронском микроскопијом Калцинисани узорци
су синтеровани на 1000 degC са задржавањем од 3 сата у атмосфери ваздуха (CaP1000
CaMgP1000 and CaSrNaP1000) Синтеровани материјали окарактерисани су рендгенском
дифракцијом на поликристалном узорку инфрацрвеном спекторскопијом са
Фуријеовом трансформацијом и скенирајућом електронском микроскопијом са
M Mirković et alScience of Sintering 50 (2018) 95-109 ___________________________________________________________________________
109
енергетско дисперзивном анализом као и методе комплексне импендансе Највећа
проводљивост добијена је на температури од 700 degC и износи 190times10-3
Ω-1
cm-1
за
мешовито допирани фосфатни материјал (CaSrNaP1000) Добијене активационе
енергије проводљивости износе 031 eV у температурном опсегу од 500-700 ordmC
Кључне речи Синтеровање Рендгенска дифракција Електрична проводљивост
Фосфатни материјали Горивне ћелије
copy 2016 Authors Published by the International Institute for the Science of Sintering This
article is an open access article distributed under the terms and conditions of the Creative
Commons mdash Attribution 40 International license
(httpscreativecommonsorglicensesby40)
M Mirković et al Science of Sintering 50 (2018) 95-109
___________________________________________________________________________
96
hydrothermal synthesis [14] and solid state reaction method [15] In comparison to these
methods direct precipitation from aqueous solution provides an easier way of preparing HA
ceramic and at the same time provides a large amount high purity material [16-18]
Furthermore ion substituted calcium phosphate materials can be prepared using
simple chemical precipitation method which is of great importance when it comes to
obtaining a solid ionic conductor for use in ITndashSOFCs the advantages of this method include
simple synthesis low cost and ability to obtain large quantity and high purity of solid
electrolyte [18]
In recent years ionic substitutions of Mg2+
Na+ and Sr
2+ in HA [11 12] proved very
interesting for their potential implementation in ITndashSOFCs owing to the morphology
thermal stability and structural and mechanical properties [19] Lowndashtemperature treatment
(400ndash500 ordmC) allows preserving the particle morphology and developing nanoscale structures
ie grains [20 21] In order to obtain compact material densification process usually requires
sintering at temperatures at or above 1000 ordmC however this leads to the formation of high
temperature calcium phosphate ceramics [22-24] Knowledge of thermal stability ionic
transport properties and possible mechanisms of conduction properties of different calcium
phosphate materials provides a key for their application in ITndashSOFCs [11 12] In this study
we show Mg2+
Na+ and Sr
2+ HA materials as suiTab candidates for ionic conductive
materials Materials were treated thermally in air atmosphere at 400 ordmC (CaP400 MgCaP400 and
NaSrCaP400) Additionally behavior doped CaP after sintering at 1000 ordmC during 3 h (CaP1000
MgCaP1000 and NaSrCaP1000) was studied The main objective was to obtain a range of dense
solid solution calcium phosphate materials with suiTab morphology and grain size for the
application in Intermediate Temperature Solid Oxide Fuel Cells
2 Material and methods
21 Synthesis of CaP CaMgP and CaSrNaP samples
Samples were prepared by solution precipitation method Pure hydroxyapatite (CaP)
was prepared by drop wise addition of 250 ml 03 M NaH2PO4+H2O into 250 ml Ca(OH)2
05 M solution which was magnetically stirred The pH was adjusted to 11 using NH4OH at
temperature about 100 ordmC To synthesize Mg (CaMgP) and NaSr (NaSrCaP) substituted
hydroxyapatite materials we used slightly modified precipitation method To obtain MgHA
250 ml NaH2PO4+H2O 03 M was heated at 100 ordmC and added drop wise into mechanically
stirred mixture of 250 ml Ca(OH)2 05 M and 250 ml MgCl2∙6H2O 025 M heated to about
100 ordmC pH was adjusted to 12 using NH4OH solution For NaSrCaP synthesis mixture of 250
ml of (CH3COOH)2Ca∙H2O 003 M and 250 ml Sr(CH3COO)2 003 M was added drop wise
into 500 ml NaH2PO4+H2O 03 M at room temperature The precipitates were washed
centrifuged at 3000 rpm and dried at 70 ordmC for 17 h Dry powders were thermally treated at
400 ordmC during 3h in air atmosphere and then uniaxially pressed at 105 MPa into compact
pelets (8 mm diameter) and sintered at 1000 ordmC for 3 h in air atmosphere
22 Densification of CaP CaMgP and CaSrNaP samples
Samples were uniaxially pressed at 105 MPa and thermally treated Sample annealing
was carried out at 1000 ordmC for 3 h in an air at a heating rate of 5 ordmCmin The resulting density
was determined via Archimedesrsquo method Densified samples were characterized using X-ray
diffraction (XRD) scanning electron microscopy (SEM) Energy Dispersive Spectroscopy
(EDS) and complex impedance methods
M Mirković et alScience of Sintering 50 (2018) 95-109 ___________________________________________________________________________
97
23 Characterization of CaP CaMgP and CaSrNaP samples
Phase composition before and after thermal treatment as well as crystallite size
evolution were analyzed by X-ray diffraction (XRD) using Ultima IV Rigaku diffractometer
equipped with CuKα radiation using a generator voltage 400 kV and a generator current 400
mA The range of 5ndash80deg 2θ was used in a continuous scan mode with a scanning step size of
002deg and count time 2degmin Analysis was done using PDXL2 software (version 2030)
[25] with reference to ICDD database [26] For quantitative phase analysis RIR method was
used Structural modeling was done using VESTA program [27] Spectroscopic studies of the
synthesized and thermally treated materials were carried out in the mid infrared (MIR)
regions (4000ndash400 cm-1
) using Fourier transforms infrared (FTIR) spectroscopy in
transmission mode by a Perkin Elmer Spectrum Two FT-IR spectrometer using the pressed
KBr compacts (1100) technique
The morphology and chemical composition of samples were identified using a JEOL
JSM-6610LV Scanning Electron Microscope with an X-Max Energy Dispersive
Spectrometer Samples were coated with gold using a BALTEC-SCD-005 sputter coating
device and the results were recorded under high vacuum conditions The electrical
conductivity of sintered samples was measured by complex impedance method in a
frequency range 1 Hz-01 MHz using Interface 1000 Potentiostat Galvanostat ZRA and
EIS300 Electrochemical Impedance Spectroscopy Software The measurements were
conducted in air in the temperature range 500-700 ordmC with a 50 ordmC increment The amplitude
of the applied sinusoidal voltage signal was 20 mV Thin layer of high conductivity silver
paste was applied onto both sides of the sample pellets in order to provide good electrical
contact between electrolyte and electrodes The samples were placed between the silver plates
in a ceramic holder which was heated by vertical oven A PtndashRh thermocouple located just
below the bottom silver plate was used for temperature monitoring The impedance plots
obtained experimentally were fitted by means of the software ZViews for Windows (Version
32b) The resistance values were determined from the impedance diagrams recorded at
various temperatures The specific conductance was calculated from the resistance data using
the dimensions of the sample pellets
3 Results and discussion
31 Structural and morphological properties of CaP CaMgP and CaSrNaP
samples
311 XRD analysis
X-Ray diffraction patterns of synthesized pure hydroxyapatite (CaP) powders
calcined at 400 ordmC (CaP400) and sintered at 1000 ordmC (CaP1000) during 3 h are shown in Fig 1
CaP powder shows broad peaks typical for hydroxyapatite obtained by precipitation method
CaP and CaP400 crystallized as hexagonal Ca5(PO4)3OH sg P63m (ICDD 01-071-5048)
XRD results show the formation of another calcium phosphate phase in the sintered CaP1000
sample Based on peak identification structural refinement and decomposition calculation
using PDXL2 software we found that CaP1000 consists of 85 β - Ca2P2O7 (ICDD 01-071-
2123) phase and of 15 hydroxylapatite phase (ICDD 01-071-5048)
XRD diffraction patterns of Mg doped HA are shown in Fig 2 Analyses show almost
identical patterns as pure HA powder however peaks are shifted compared to pure HA
material (Fig 1) suggesting that Mg2+
cations were incorporated in the structure during
thermal treatment at 400degC XRD of CaMgP1000 sample shows that heating to 1000 ordmC leads to
the formation of high temperature magnesium phosphate phases Mg2P2O7 (ICDD 00-008-
0038 marked as open circles) and β-Ca2P2O7 (ICDD 01-071-2123 marked as solid circles)
M Mirković et al Science of Sintering 50 (2018) 95-109
___________________________________________________________________________
98
It is evident that addition of magnesium to the HA structure leads to the formation of high
temperature solid solution that includes high temperature CaMgP phases and hydroxyapatite
CaMgP100 nominal phase composition calculated using PDXL2 software showed 55 β-
Ca2P2O7 30 Mg2P2O7 and 15 hydroxyapatite phase
Fig 1 XRD patterns of CaPndashhydroxyapatite synthesized sample CaP400ndashhydroxyapatite
calcinated at 400 ordmC for 3 h in air and CaP1000ndashsintered at 1000 ordmC for 3 h in air
Fig 2 XRD patterns of CaMgPndashsynthesized sample CaMgP400ndashcalcined at 400 ordmC for 3 h in
air and CaMgP1000ndashsintered at 1000 ordmC for 3 h in air
The diffraction pattern of powder CaSrNaP (Fig 3) reveals that the dried precipitate exhibits
poor crystallinity peaks around 10ordm and 18deg 2θ (marked as inverted triangles) indicate
secondary phase with chemical composition Sr3(P3O9)2(H2O)7 (ICDD01-070-0007) The
secondary phase existence is common during precipitation method of nano material synthesis
[28] However there is no evidence of the existence of secondary phase in thermally treated
CaSrNaP400 (Fig 3) CaSrNaP XRD peaks are slightly shifted to the left implying a larger unit
cell most likely due to the incorporation of Sr into the HA lattice (Fig 3) Thermal treatment
of hydroxylapatite materials below 700 ordmC leads to surface reduction and single phase particle
coalescence without densification [24 29] At 1000 degC high temperature solid solution forms
noted as CaSrNaP1000 Sintering at this temperature with retention time of 3 h high
M Mirković et alScience of Sintering 50 (2018) 95-109 ___________________________________________________________________________
99
temperature calcium and strontium and sodium phosphate materials developed It is evident
that peaks belongs to three different phosphate phases β ndash Ca2P2O7 (01-071-2123) α-Sr2P2O7
(ICDD 01-072-149) and Na4P2O7 (ICDD 01-073-5982) Strontium and sodium phosphate
phases are orthorhombic while the calcium phosphate phase is tetragonal Structural changes
and phase transitions during sintering (temperatures above 700 degC) can be explained by the
loss of hydroxyl ions from the hydroxyapatite structure at temperatures around and higher
than 1000 degC and the formation of orthorhombic and tetragonal phosphate phases [24 30]
The peaks on XRD diagram are narrow and well defined indicating high structural
arrangement of sintered material
Quantitative phase analysis of CaSrNaP material showed 74 of α-Sr2P2O7 16 of β
ndash Ca2P2O7 and 10 of Na4P2O7 Hydroxylapatite is still evident at high temperatures in the
case of pure HA and HA doped with Mg On the other hand in the case high temperature
treatment (1000 degC for 3h) of multi-doped HA leads to the formation of mixed CaSrNaP
phase with high structural arrangement
Fig 3 XRD patterns of CaSrNaPndashsynthesized sample CaSrNaP400ndashcalcined at 400 ordmC for 3 h
in air and CaSrNaP1000ndashsintered at 1000 ordmC for 3 h in air
Tab I Unit cell parameters and crystallite sizes of obtained materials
Samples Cell parameter (Aring)
Crystallite size (nm) a c
CaP 938(14) 692(19) 14(13)
CaP400 939(10) 691(12) 152(5)
CaMgP 940(4) 687(5) 72(10)
CaMgP400 935(3) 688(3) 156(9)
CaSrNaP 934(4) 679(4) 43(5)
CaSrNaP400 945(6) 729(7) 79(14)
M Mirković et al Science of Sintering 50 (2018) 95-109
___________________________________________________________________________
100
Tab I shows variation of lattice parameters due to structural substitution during
precipitation and calcinations at 400 degC Unit cell parameters and crystallite size calculations
of were performed using cif files from American Mineralogist Crystal Data Structure Base
(AMCDSB)[31] (Tab I)
There is no significant difference in the average crystallite size of CaP and CaP400
samples (Tab I) Unit cell parameters of both samples are in similar to pure hydroxyapatite
(ICDD 01-071-5048) With the increase of temperature unit cell parameter a increases while
the parameter c decreases Average crystallite size of CaMgP material increasing with
temperature which is consistent with peak shapes of obtained material The unit cell
parameters of CaMgP are higher than of CaP material which is in correlation with published
data [32] Average crystallite size of thermally treated samples is slightly higher than
synthesized Factors that influence peak broadening are the presence of amorphous phase
during precipitation and a number of dislocations between crystallite boundaries [16 33]
During calcination at 400 ordmC the material becomes crystalline and thermodynamically sTab
allowing for better arrangement and growth of crystallites [34] The unit cell parameters of
CaSrNaP and CaSrNaP1000 have the highest value out of all examined materials as the ionic
radii of Sr2+
(113 Aring) and Na+ (152 Aring) are larger than ionic radii Ca
2+ (099 Aring) and Mg
2+
(069 Aring) Anisotropic change in unit cell dimensions could be attributed to the replacement of
Ca2+
with other cations ie Mg2+
Na+ and Sr
2+ in structure [35 36]
312 FTIR analysis
FTIR spectra of untreated (synthesized) samples CaP CaMgP and CaSrNaP (Fig 4)
showed the presence of characteristic apatite signals (around 563 610 657 1030 1077 cm-1
)
Fig 4b [36] The absorbed water band is around 3500 cm-1
and the band at 1413 corresponds
to CO32-
group vibration [18] In comparison with FTIR of stoichiometric HA bands of
CaSrNaP and CaMgP are red-shifted because of the presence of foreign ions incorporated into
the structure [37-39] Absence of the 631 cm-1
band that belongs to vibration modes of the
apatitic OH groups in all tree spectra most likely indicates a lack of hydroxyl content and a
possible substitution of hydroxyl groups by carbonate groups [37]
Fig 4 FTIR spectra of untreated (synthesized) samples a) CaMgP b) CaP and c) CaSrNaP
M Mirković et alScience of Sintering 50 (2018) 95-109 ___________________________________________________________________________
101
Furthermore the FTIR spectrum of CaMgP shows a slight indication of 1410 cm-1
carbonate band (Fig 4a) thus indicating no significant presence of CO3 group in CaMgP
powder [40] Weak band of MgHCO3 can be noticed around 1634 cm-1
[36] CaSrNaP band at
1385 cm-1
indicates incorporation of Sr2+
and Na+ ions into the structure of hydroxylapatite
(Fig 4c) [35 38] A band that indicates carbonate group (coming from the reaction medium)
in the structure of CaSrNaP is demonstrated by bands at 1417 and 1639 cm-1
[41] FTIR
spectra confirm previously described XRD results that using acetate solution multi doped
hydroxyapatite material can be obtained
FTIR spectra of thermally treated samples at 1000 ordmC for 3 h in air are shown in Fig
5 also shows presence of the characteristic signals of apatite [36] CaMgP1000 (Fig 5a)
sample shows bands in spectral regions 1470-870 cm-1
and 605-560 cm-1
which are generally
related to P-O species with different phosphorous configuration [42 43] Characteristic
vibrations of Mg2P2O7 phase are bands at 1050 605 and 560 cm-1
[42] The bands at around
1050 cm-1
belong to triply degenerate asymmetric P-O stretching mode while the band
around 955 cm-1
belongs to P-O stretching mode these bands belong to β-Ca2P2O7 phase [43]
There is no evidence of any characteristic hydroxyl group vibrations (631 and 3572 cm-1
)
CaP1000 (Fig 5b) bands are weaker than in untreated samples indicating lack of water
vibrations in structure The bands around 600 and 580 cm-1
are attributed to different types of
P-O-P bending mode vibrations which belong to β-Ca2P2O7 [43] Figure 5c shows vibration
spectrum of CaSrNaP1000 material Presence of previously described P-O and P-O-P vibrations
which belong to β-Ca2P2O7 phase also belong to Sr2P2O7 [43 44] The band around 740 cm-1
belongs to P-O-P bridge stretching vibration a striking feature of (P2O7)4-
ions in Sr2P2O7
FTIR spectrum of CaSrNaP (Fig 5c) shows no evidence of O-H stretching modes at around
3500 and 1600 cm-1
[44] Metal-oxygen vibrations in these types of systems are generally too
low to be seen in the 400-4000 cm-1
region
Fig 5 FTIR specta of thermally treated samples at 1000 degC in air for 3 h a) CaMgP1000 b)
CaP1000 and c) CaSrNaP1000
M Mirković et al Science of Sintering 50 (2018) 95-109
___________________________________________________________________________
102
Based on FTIR analysis at 1000 degC high temperature CaMgP1000 material is obtained
All of identified vibrations that belong to CaP1000 material (Fig 5b) confirm that β-Ca2P2O7 is
obtained by sintering hydroxyapatite for 3h on 1000degC in air atmosphere The analysis of
CaSrNaP1000 spectrum (Fig 5c) confirms absence of O-H vibrations and presence of (P2O7)4-
vibrations which is unique for X2P2O7 types of compounds [43 44]
313 Morphology (SEM analysis) and element composition (EDS analysis)
SEM analysis confirms that all obtained samples using precipitation method have
crystallite size of nanometer dimensions In addition SEM images (Fig 6) show that the
particle size in all samples increases with the increase of temperature Namely nanoparticles
have a natural tendency to agglomerate for two reasons firstlyminusthe agglomeration is a more
sTab configuration from an energetic point of view and secondlyminusthe agglomeration allows
the crystallite growth [45 46]
Fig 6 SEM micrographs of A) CaP CaP400 and CaP1000 B) CaMgP CaMgP400 and
CaMgP1000 C) CaSrNaP CaSrNaP400 and CaSrNaP1000 with EDS spectra
Sintering at 1000 ordmC in air for 3 h led to a higher degree of densification of
CaSrNaP1000 (98 TD) than of CaMgP1000 (86 TD) The main reason for the differences in
density may be pore and particle size distribution ie different morphology (Fig 6) The
samples after calcinations at 400 ordmC during 3 h show more uniform particles and well ordered
structure Calcinations was done in order to allow better packing and lower inner activity and
it was expected that these samples may achieve higher density after sintering This was
achieved for CaSrNaP1000 sample where the grains are uniformly sintered and the contacts
between grains show hexagonal forms and the inserted section clearly demonstrates that we
have indeed obtained dense high temperature CaSrNaP material (Fig 6c) On the other hand
CaMgP1000 (Fig 6b) exhibits grain growth due to formation of agglomerated grains
Densification retards or inhibits wide pore distribution during densification the big pores
M Mirković et alScience of Sintering 50 (2018) 95-109 ___________________________________________________________________________
103
became larger while small pores shrink and disappear [47 48] SEM micrograph of
CaMgP1000 (Fig 6b) shows numerous agglomerates of hexagonal particles with plate like
morphology bonded together with average grain size between 2 and 5 microm with non-uniform
particles with many vacancies less compact particle packing higher inner activity To
confirm the chemical composition the EDS microanalysis was done on the CaP1000
CaMgP1000 and CaSrNaP1000 sintered samples (Fig 6) It revealed that chemical composition
very similar to nominal chemical composition According to semi quantitative chemical
analysis the mean values for the ratios CaPO for CaP1000 sample determined from four runs
are 21275 These ratios correspond to empirically calculated compound with nominal
composition-(Ca21P2O75) The mean value of the chemical ratios CaMgPO for CaMgP1000
was also determined from four runs and amounts to 120727 which corresponds to
empirical compound with following composition-(Ca12Mg07)19P2O7 The mean value of the
chemical ratios Ca Sr Na PO of the NaSrHA1000 sample was determined from four runs
and amounts to 0506227 which corresponds to-Ca05Sr06Na2P2O7 These results (Fig 6)
confirm the assumption that sintered samples based on hydroxyapatite at high temperature
(1000 ordmC) remain sTab and almost nominal chemical composition The obtained EDS results
are in good agreement with XRD results
32 Electrical conductivity
Recently ionic conductors with the apatite type structures attracted a great deal of
attention as an alternative electrolytes to the conventional yttria-stabilized zirconia to operate
in ITndashSOFCs in the intermediate temperature range (600-800 ordmC) [49-52] Namely
hydroxyapatite belongs to apatitic group of phosphate minerals with general formula
Me10(XO4)6Y2 with ionic substitution ability [53] Based on XRD (Tab I) unit cell
parameters of CaMgP samples are slightly increased compared to pure CaP sample the
contraction of a and c parameter can be attributed to much smaller ionic radius of Mg2+
than
Ca2+
[54] The change of unit cell parameters a and c of CaSrNaP indicate that Sr2+
and Na+
ions are incorporated in structure and not just adhered to the crystal where Sr2+
will firstly
replace Ca2+
ion in Ca2 site [55] On the other hand Na+ ion as a smaller than Ca
2+ could
possible replace Ca2+
in Ca1 site while a vacancy may formed at the OH- site [56] At 1000degC
structural transformation of hydroxyapatite occurs and it transforms to a high temperature
mixed phosphate ceramics with M2X2O7 structure where X ions are tetrahedral coordinated
(Fig 7)
Fig 7 Structures of M2X2O7 a) αndashSr2P2O7 b) βndashCa2P2O7 and c) Na4P2O7 (the green spheres
are Sr atoms blue spheres are Ca atoms the yellow spheres are Na atoms the red spheres are
O atoms and the purple spheres are P atoms)
The crystal structures of calcium strontium and sodium phosphate materials are
comprised of diphosphate ions which are linked to metallic ions to form a three
M Mirković et al Science of Sintering 50 (2018) 95-109
___________________________________________________________________________
104
dimensional network in β ndash Ca2P2O7 α-Sr2P2O7 and Na4P2O7 structures [57 58] A
common characteristic of thermal behavior of all phosphates is that under thermal
treatment they undergo dehydratation-condensation reaction which leads to phosphate
tetrahedral bonded to mutual oxygen atom The ion conductivity in these materials
occurs mainly via O- interstitials with preferential cndashaxis conduction [50 51 56 59]
in comparison to O2-
vacancy migration in fluorite and perovskitendashbased electrolytes
[59-63] Generally the impedance spectra are presented as negative of imaginary component
of impedance (ndashZimag) versus real component of impedance (Zreal) ie as Nyquist plot The
semicircles at high and intermediate frequency are ascribed to bulk and grain boundary
processes respectively while semicircle at low frequency represents the electrode process
contribution [56] In our case for the potential application in IT-SOFCs the measurements of
ionic conductivity of electrolytes in solid state of the CaMgP1000 and CaSrNaP1000 samples
were done in intermediate temperature range of 500-700 ordmC with the increments of 50 ordmC
The original Nyquist plots recorded in the available frequency range (1 Hz-100 kHz) are
presented in Fig 8 As it could be seen (Fig 8a and 8b) with increasing the temperature the
values of both resistance elements (Rb and Rgb) obviously decrease which causes an increase
in ωmax Consequently the whole region of the impedance points shifts towards the lowndash
frequency semicircle and at higher temperatures instead of Rb and Rgb separately only the
whole sum Rb+Rgb became readable in the available frequency range In this case at higher
temperatures the time constants associated with the bulk and grain boundary impedances are
much lower than those associated with the electrode interface As a result semicircles due to
bulk and grain boundary disappear and only a single semicircle due to electrode interfacial
processes can be observed [64 66] Therefore only the whole sum Rb+Rig became readable
and the values of total resistance were estimated from the experimental cross section of
obtained semicircles with the real component of impedance (Zreal) this intercept is marked by
arrows in Fig 8 (inset) New semicircles are observed to appear in a lowndashfrequency region
being particularly visible in temperature range at 650-700 ordmC (Fig 8b) Almost doubtless it
originates from the oxygen electrode reactions O2O2-
[64] which does not belong to the
scope of this study
20 30 40 50 60 700
10
20
30
40
50
60
70 a)
20 30 40 50 60 700
1
2
3
4
5
6
7
8
9
10x103
x103
-Zim
ag
(c
m)
Zreal (cm)
x102
x102
500 C
550 C
600 C
-Zim
ag
(c
m)
Zreal (cm)
6 8 10 12 14 160
2
4
6
8
10
12
14
16 b)x102
6 8 10 12 14 160
2
x102
x102
-Zim
ag
(c
m)
Zreal (cm)
x102
650 C
700 C
-Zim
ag
(c
m)
Zreal (cm)
Fig 8 (a)(b) Complex impedance plots of the sintered samples CaSrNaP1000 measured in the
temperature range from 500 to 700 ordmC in air atmosphere The arrows indicate the points on
the real axis corresponding to the readings Rb+Rgb
The total ionic conductivity of the sintered CaSrNaP1000 sample is shown in Tab II Obtained
values (at 700 ordmC amount to 418times10-3
Ω-1
cm-1
) are slightly higher than previously published
ones [64 66] More specifically values of ionic conductivity observed at 700 ordmC in this work
M Mirković et alScience of Sintering 50 (2018) 95-109 ___________________________________________________________________________
105
are similar to values obtained at 800 and 900 ordmC [67-71] This holds also if we compare the
results we found at lower temperatures with the literature data [64-67]
Tab II The temperature dependence of total ionic conductivity (κ) of the sintered CaMgP1000
and CaSrNaP1000 sample
COMPOSITION
κ
(Ω-1
cm-1
)
500 ordmC
κ
(Ω-1
cm-1
)
550 ordmC
κ
(Ω-1
cm-1
)
600 ordmC
κ
(Ω-1
cm-1
)
650 ordmC
κ
(Ω-1
cm-1
)
700 ordmC
CaMgP1000 123times10-4
222times10-4
358times10-4
294times10-4
312times10-4
CaSrNaP1000 227times10-4
356times10-4
657times10-4
117times10-3
190times10-3
According to the results listed in Tab II the dependence log κ = f(1T) of the sintered
sample CaSrNaP1000 is presented in Fig 9 The activation energy of conductivity (Ea) was
calculated from the Arrhenius plot according to the equation
Tk
EAT a 1
lnln (1)
where σ is the conductivity T is the absolute temperature A is the pre-exponential factor and
k is the Boltzmann constant Activation energy of total conductivity for sintered CaSrNaP1000
sample is 031 eV Ea values presented in our work are much lower than previously found [67
68 70 71] In addition comparing the obtained values of Ea with activation energies of
similar oxygen ion conductors containing dopants [71] obtained Ea values were significantly
lower We suggest that this is a consequence of wellndashordered structure and better process
ability of nanopowders obtained application method
100 105 110 115 120 125 130-38
-36
-34
-32
-30
-28
-26
Ea = 031 eV
log
k
1000T (K-1)
Fig 9 The dependence log κ = f(1T) of the sintered CaSrNaP1000 sample
M Mirković et al Science of Sintering 50 (2018) 95-109
___________________________________________________________________________
106
4 Conclusions
The pure Ca and doped with Mg and NaSr hydroxyapatite (CaP CaMgP and NaSrP)
were successfully synthesized using slightly modified solution precipitation method (partial
substitution nitrates by acetate solutions) and thus we achieved higher solubility and more
cost effective synthesis Any secondary phase that formed during synthesis decomposed
during thermal treatment leaving pure CaP CaMgP and CaSrNaP hexagonal (P63m)
Ca5(PO4)3OH materials at room temperature During sintering process at 1000 ordmC in an air for
3 h we developed high temperature doped calcium phosphate materials ie CaMgP1000 and
CaSrNaP1000 materials SEM analysis showed that CaMgP1000 and CaSrNaP1000 materials have
high structural arrangement EDS analysis confirmed that we obtained phosphate compound
type-M2X2O7 The fine crystalline powders after sintering at 1000 ordmC achieved 98 of
theoretical density The maximum value of electrical conductivity at 700 ordmC was achieved for
CaSrNaP1000 material with amounts to 190times10-3
Ω-1
cm-1
Electrical characteristics of the
sintered samples indicate the superiority of solid solution for the application in SOFCs
Additionally considering high sintering temperature one can conclude that the obtained
phosphate material demonstrates high thermal stability an important characteristics in its
potential use as solid electrolyte However it is important to note that the future research
should include furthering lower temperature synthesis routes improving stability ITndashSOFCs
electrolyteelectrode molecular dynamics (MD) simulations of conduction mechanisms and
the general search for improved ion conductors Also the breadth of possible doping regimes
in this phosphate material provides new opportunities to design and optimize their transport
parameters for potential clean energy applications
Acknowledgements
This work was supported by the Ministry of Education Science and Technological
development of Serbia as part of Projects III 45012 and OI 176016
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Bogdanović Mat Chem Phys 153 (2015) 422
10 M Stojmenović M Žunić J Gulicovski D Bajuk-Bogdanović I Holclajtner-
Antunović V Dodevski J Mater Sci 50 (2015) 3781
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107
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28 (1988) 660
13 F Bakan O Laccedilin H Sarac Powder Technol 233 (2013) 295
14 M Sadat-Shojai M-T Khorasani A Jamshidi J Cryst Growth 361 (2012) 73
15 S Pramanik A K Agarwal K N Rai A Garg Ceram Int 33 (2007) 419
16 I Mobasherpour MS Heshajin A Kazemzadeh M Zakeri J Alloys Comp 430
(2007) 330
17 A K Nayak Int Jour ChemTech Res 2 (2010) 903
18 T T Trang Pham T Phuong Nguyen T Nam Pham T Phuong Vu D L Tran
HThai Vietnam Academy of Science and Technology 4 (2013) 1
19 S C Cox P Jamshidi L M Grover K K Mallick Mat Sci Eng C-Bio S 35
(2014) 106
20 E Champion Acta Biomater 9 (2013) 5855
21 S J Kalita A Bhardwaj H A Bhatt Mat Sci Eng C-Bio S 27 (2007) 441
22 S Ramesh K L Aw R Tolouei M Amiriyan C Y Tan M Hamdi Ceram Int
39 (2013) 111
23 Đ Veljović G Vuković I Steins E Palcevskis P S Usković R Petrović Sci
Sinter 45 (2013) 233
24 S Raynaud E Champion D Bernache-Assollant Biomater 23 (2002) 1073
25 Rigaku Corporation Tokyo J PDXL Version 2030 Integrated X-ray Powder
Diffraction Software 2011
26 Powder Diffraction File P-D announcement of new data-base release 2012
International Centrefor Diffraction Data (ICDD)
27 K Momma F Izumi J Appl Crystallogr 44 (2011) 1272
28 M Prekajski M Mirković B Todorović A Matković M Marinović-Cincović J
Luković B Matović J Eur CeramSoc 36 (2016) 1293
29 J Wang LL Shaw Adv Mater 19 (2007) 2364
30 A Mortier J Lemaitre P Rouxet Thermochim Acta 113 (1987) 133
31 R T Downs M Hall-Wallace Am Mineralog 88 (2003) 247
32 G Qi S Zhang K A Khor C Liu X Zeng W Weng Thin Solid Films 516
(2008) 5176
33 S C Cox P Jamshidi L M Grover K K Mallick Mat Sci Eng C-Bio S 35
(2014) 106
34 A Cuccu S Montinaro R Orrugrave G Cao D Bellucci A Sola Ceram Int 41
(2015)725
35 W Zhang Y Shen H Pan K Lin X Liu B W Darvell Acta Biomater 7 (2011)
800
36 J A Gadsden Infrared spectra of minerals and related inorganic compounds
Butterworths London 1975
37 M Iafisco A Ruffini A Adamiano S Sprio A Tampieri Mat Sci Eng C-Bio S
35 (2014) 212
38 S Koutsopoulos J Biomed Mater Res 62 (2002) 600
39 S-C Liou S-Y Chen H-Y Lee J-S Bow Biomaterials 25 (2004) 189
40 M E Fleet Biomaterials 30 (2009) 1473
41 N V Chukanov Infrared spectra of mineral species Extended library Springer
London Limited 2013
42 V Koleva V Stefov M Najdoski A Cahil Thermochim Acta 619 (2015) 20
43 B Mehdikhani G H Borhani J Ceram Process Res 16 (2015) 308
44 P Solanki S Vasant M Joshi Int J Appl Ceram Tech 11 (2014) 663
45 G B Balazs R Glass Solid State Ionics 76 (1995) 155
46 G Buvaneswari K Valsalan Mater Lett 134 (2014) 252
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47 A Farzadi F Bakhshi M Solati-Hashjin M Asadi-Eydivand N A Osman Ceram
Int40 (2014) 6021
48 A G Evans C H Hsueh J Am Ceram Soc 69 (1986) 444
49 A J Jacobson Chem Mater 22 (2010) 660
50 A Orera P R Slater Chem Mater 22 (2010) 675
51 L Malavasi C A J Fisher M S Islam Chem Soc Review 39 (2010) 4370
52 D Marrero-Loacutepez M C Martiacuten-Sedentildeo J Pentildea-Martiacutenez J C Ruiz-Morales P
Nuacutentildeez M A G Aranda J Power Sources 195 (2010) 2496
53 M P Moreira G D de Almeida Soares J Dentzer K Anselme L Aacute de Sena A
Kuznetsov Mat Sci Eng C-Bio S 61 (2016) 736
54 F Ren Y Leng R Xin X Ge Acta Biomater 6 (2010) 2787
55 Z Y Li W M Lam C Yang B Xu G X Ni S A Abbah Biomaterials 28
(2007)1452
56 Z Leilei L Hejun L Kezhi F Qiangang Z Yulei L Shoujie Ceram Int 40
(2014) 13123
57 K Y Leung C Calvo Can J Chem 50 (1972) 2519
58 H Larsove J Ingela M Christina Acta Chemica Scandinavica 22 (1968) 1419
59 E Kendrick M S Islam P R Slater J Mater Chem 17 (2007) 3104
60 E Kendrick M S Islam P R Slater Chem Commun 715 (2008) 7
61 K Fukuda T Asaka R Hamaguchi T Suzuki H Oka A Berghout Chem Mater
23 (2011) 5474
62 E Kendrick M S Islam P R Slater Solid State Ionics 177 (2007) 3411
63 L Leon-Reina E R Losilla M Martinez-Lara S Bruque M A G J Mater
Chem 14 (2004) 1142
64 N Nallamuthu I Prakash N Satyanarayana M Venkateswarlu J Alloys and
Compounds 509 (2011) 1138
65 M Sato Y Kono H Ueda K Uematsu K Toda Solid State Ionics 83 (1996)249
66 D Marrero-Loacutepez L dos Santos-Goacutemez L Leoacuten-Reina J Canales-Vaacutezquez E R
Losilla Journal of Power Sources 245 (2014) 107
67 S Nakayama T Kageyama H Aono Y Sadaoka J Mater Chem 5 (1995) 1801
68 Y Masubuchi M Higuchi T Takeda S Kikkawa J Alloys and Compounds 408
(2006) 641
69 J McFarlane S Barth M Swaffer J E H Sansom P R Slater Ionics 8 (2002)
149
70 S Tao J T S Irvine Mater ResBullet 36 (2001) 1245
71 J E H Sansom J R Tolchard P R Slater M S Islam Solid State Ionics 167
(2004) 17
Садржај Метода таложне титрације коришћена је за добијање чистог и допираног
хидроксиапатита јонима Sr Mg и Na (CaP CaMgP and CaSrNaP) За синтезу
супституисаних хидроксиапатитских материјала коришћена је модификована
таложна метода у односу на досада познате методе Због боље растворљивости и
знатно веће исплативости коришћени су раствори ацетата уместо нитрата
Синтетисани материјали су калцинисани на 400 degC а затим окарактерисани
рендгенском дифракцијом инфра црвеном спектроскопијом са Фуријеовом
трансформацијом и скенирајућом електронском микроскопијом Калцинисани узорци
су синтеровани на 1000 degC са задржавањем од 3 сата у атмосфери ваздуха (CaP1000
CaMgP1000 and CaSrNaP1000) Синтеровани материјали окарактерисани су рендгенском
дифракцијом на поликристалном узорку инфрацрвеном спекторскопијом са
Фуријеовом трансформацијом и скенирајућом електронском микроскопијом са
M Mirković et alScience of Sintering 50 (2018) 95-109 ___________________________________________________________________________
109
енергетско дисперзивном анализом као и методе комплексне импендансе Највећа
проводљивост добијена је на температури од 700 degC и износи 190times10-3
Ω-1
cm-1
за
мешовито допирани фосфатни материјал (CaSrNaP1000) Добијене активационе
енергије проводљивости износе 031 eV у температурном опсегу од 500-700 ordmC
Кључне речи Синтеровање Рендгенска дифракција Електрична проводљивост
Фосфатни материјали Горивне ћелије
copy 2016 Authors Published by the International Institute for the Science of Sintering This
article is an open access article distributed under the terms and conditions of the Creative
Commons mdash Attribution 40 International license
(httpscreativecommonsorglicensesby40)
M Mirković et alScience of Sintering 50 (2018) 95-109 ___________________________________________________________________________
97
23 Characterization of CaP CaMgP and CaSrNaP samples
Phase composition before and after thermal treatment as well as crystallite size
evolution were analyzed by X-ray diffraction (XRD) using Ultima IV Rigaku diffractometer
equipped with CuKα radiation using a generator voltage 400 kV and a generator current 400
mA The range of 5ndash80deg 2θ was used in a continuous scan mode with a scanning step size of
002deg and count time 2degmin Analysis was done using PDXL2 software (version 2030)
[25] with reference to ICDD database [26] For quantitative phase analysis RIR method was
used Structural modeling was done using VESTA program [27] Spectroscopic studies of the
synthesized and thermally treated materials were carried out in the mid infrared (MIR)
regions (4000ndash400 cm-1
) using Fourier transforms infrared (FTIR) spectroscopy in
transmission mode by a Perkin Elmer Spectrum Two FT-IR spectrometer using the pressed
KBr compacts (1100) technique
The morphology and chemical composition of samples were identified using a JEOL
JSM-6610LV Scanning Electron Microscope with an X-Max Energy Dispersive
Spectrometer Samples were coated with gold using a BALTEC-SCD-005 sputter coating
device and the results were recorded under high vacuum conditions The electrical
conductivity of sintered samples was measured by complex impedance method in a
frequency range 1 Hz-01 MHz using Interface 1000 Potentiostat Galvanostat ZRA and
EIS300 Electrochemical Impedance Spectroscopy Software The measurements were
conducted in air in the temperature range 500-700 ordmC with a 50 ordmC increment The amplitude
of the applied sinusoidal voltage signal was 20 mV Thin layer of high conductivity silver
paste was applied onto both sides of the sample pellets in order to provide good electrical
contact between electrolyte and electrodes The samples were placed between the silver plates
in a ceramic holder which was heated by vertical oven A PtndashRh thermocouple located just
below the bottom silver plate was used for temperature monitoring The impedance plots
obtained experimentally were fitted by means of the software ZViews for Windows (Version
32b) The resistance values were determined from the impedance diagrams recorded at
various temperatures The specific conductance was calculated from the resistance data using
the dimensions of the sample pellets
3 Results and discussion
31 Structural and morphological properties of CaP CaMgP and CaSrNaP
samples
311 XRD analysis
X-Ray diffraction patterns of synthesized pure hydroxyapatite (CaP) powders
calcined at 400 ordmC (CaP400) and sintered at 1000 ordmC (CaP1000) during 3 h are shown in Fig 1
CaP powder shows broad peaks typical for hydroxyapatite obtained by precipitation method
CaP and CaP400 crystallized as hexagonal Ca5(PO4)3OH sg P63m (ICDD 01-071-5048)
XRD results show the formation of another calcium phosphate phase in the sintered CaP1000
sample Based on peak identification structural refinement and decomposition calculation
using PDXL2 software we found that CaP1000 consists of 85 β - Ca2P2O7 (ICDD 01-071-
2123) phase and of 15 hydroxylapatite phase (ICDD 01-071-5048)
XRD diffraction patterns of Mg doped HA are shown in Fig 2 Analyses show almost
identical patterns as pure HA powder however peaks are shifted compared to pure HA
material (Fig 1) suggesting that Mg2+
cations were incorporated in the structure during
thermal treatment at 400degC XRD of CaMgP1000 sample shows that heating to 1000 ordmC leads to
the formation of high temperature magnesium phosphate phases Mg2P2O7 (ICDD 00-008-
0038 marked as open circles) and β-Ca2P2O7 (ICDD 01-071-2123 marked as solid circles)
M Mirković et al Science of Sintering 50 (2018) 95-109
___________________________________________________________________________
98
It is evident that addition of magnesium to the HA structure leads to the formation of high
temperature solid solution that includes high temperature CaMgP phases and hydroxyapatite
CaMgP100 nominal phase composition calculated using PDXL2 software showed 55 β-
Ca2P2O7 30 Mg2P2O7 and 15 hydroxyapatite phase
Fig 1 XRD patterns of CaPndashhydroxyapatite synthesized sample CaP400ndashhydroxyapatite
calcinated at 400 ordmC for 3 h in air and CaP1000ndashsintered at 1000 ordmC for 3 h in air
Fig 2 XRD patterns of CaMgPndashsynthesized sample CaMgP400ndashcalcined at 400 ordmC for 3 h in
air and CaMgP1000ndashsintered at 1000 ordmC for 3 h in air
The diffraction pattern of powder CaSrNaP (Fig 3) reveals that the dried precipitate exhibits
poor crystallinity peaks around 10ordm and 18deg 2θ (marked as inverted triangles) indicate
secondary phase with chemical composition Sr3(P3O9)2(H2O)7 (ICDD01-070-0007) The
secondary phase existence is common during precipitation method of nano material synthesis
[28] However there is no evidence of the existence of secondary phase in thermally treated
CaSrNaP400 (Fig 3) CaSrNaP XRD peaks are slightly shifted to the left implying a larger unit
cell most likely due to the incorporation of Sr into the HA lattice (Fig 3) Thermal treatment
of hydroxylapatite materials below 700 ordmC leads to surface reduction and single phase particle
coalescence without densification [24 29] At 1000 degC high temperature solid solution forms
noted as CaSrNaP1000 Sintering at this temperature with retention time of 3 h high
M Mirković et alScience of Sintering 50 (2018) 95-109 ___________________________________________________________________________
99
temperature calcium and strontium and sodium phosphate materials developed It is evident
that peaks belongs to three different phosphate phases β ndash Ca2P2O7 (01-071-2123) α-Sr2P2O7
(ICDD 01-072-149) and Na4P2O7 (ICDD 01-073-5982) Strontium and sodium phosphate
phases are orthorhombic while the calcium phosphate phase is tetragonal Structural changes
and phase transitions during sintering (temperatures above 700 degC) can be explained by the
loss of hydroxyl ions from the hydroxyapatite structure at temperatures around and higher
than 1000 degC and the formation of orthorhombic and tetragonal phosphate phases [24 30]
The peaks on XRD diagram are narrow and well defined indicating high structural
arrangement of sintered material
Quantitative phase analysis of CaSrNaP material showed 74 of α-Sr2P2O7 16 of β
ndash Ca2P2O7 and 10 of Na4P2O7 Hydroxylapatite is still evident at high temperatures in the
case of pure HA and HA doped with Mg On the other hand in the case high temperature
treatment (1000 degC for 3h) of multi-doped HA leads to the formation of mixed CaSrNaP
phase with high structural arrangement
Fig 3 XRD patterns of CaSrNaPndashsynthesized sample CaSrNaP400ndashcalcined at 400 ordmC for 3 h
in air and CaSrNaP1000ndashsintered at 1000 ordmC for 3 h in air
Tab I Unit cell parameters and crystallite sizes of obtained materials
Samples Cell parameter (Aring)
Crystallite size (nm) a c
CaP 938(14) 692(19) 14(13)
CaP400 939(10) 691(12) 152(5)
CaMgP 940(4) 687(5) 72(10)
CaMgP400 935(3) 688(3) 156(9)
CaSrNaP 934(4) 679(4) 43(5)
CaSrNaP400 945(6) 729(7) 79(14)
M Mirković et al Science of Sintering 50 (2018) 95-109
___________________________________________________________________________
100
Tab I shows variation of lattice parameters due to structural substitution during
precipitation and calcinations at 400 degC Unit cell parameters and crystallite size calculations
of were performed using cif files from American Mineralogist Crystal Data Structure Base
(AMCDSB)[31] (Tab I)
There is no significant difference in the average crystallite size of CaP and CaP400
samples (Tab I) Unit cell parameters of both samples are in similar to pure hydroxyapatite
(ICDD 01-071-5048) With the increase of temperature unit cell parameter a increases while
the parameter c decreases Average crystallite size of CaMgP material increasing with
temperature which is consistent with peak shapes of obtained material The unit cell
parameters of CaMgP are higher than of CaP material which is in correlation with published
data [32] Average crystallite size of thermally treated samples is slightly higher than
synthesized Factors that influence peak broadening are the presence of amorphous phase
during precipitation and a number of dislocations between crystallite boundaries [16 33]
During calcination at 400 ordmC the material becomes crystalline and thermodynamically sTab
allowing for better arrangement and growth of crystallites [34] The unit cell parameters of
CaSrNaP and CaSrNaP1000 have the highest value out of all examined materials as the ionic
radii of Sr2+
(113 Aring) and Na+ (152 Aring) are larger than ionic radii Ca
2+ (099 Aring) and Mg
2+
(069 Aring) Anisotropic change in unit cell dimensions could be attributed to the replacement of
Ca2+
with other cations ie Mg2+
Na+ and Sr
2+ in structure [35 36]
312 FTIR analysis
FTIR spectra of untreated (synthesized) samples CaP CaMgP and CaSrNaP (Fig 4)
showed the presence of characteristic apatite signals (around 563 610 657 1030 1077 cm-1
)
Fig 4b [36] The absorbed water band is around 3500 cm-1
and the band at 1413 corresponds
to CO32-
group vibration [18] In comparison with FTIR of stoichiometric HA bands of
CaSrNaP and CaMgP are red-shifted because of the presence of foreign ions incorporated into
the structure [37-39] Absence of the 631 cm-1
band that belongs to vibration modes of the
apatitic OH groups in all tree spectra most likely indicates a lack of hydroxyl content and a
possible substitution of hydroxyl groups by carbonate groups [37]
Fig 4 FTIR spectra of untreated (synthesized) samples a) CaMgP b) CaP and c) CaSrNaP
M Mirković et alScience of Sintering 50 (2018) 95-109 ___________________________________________________________________________
101
Furthermore the FTIR spectrum of CaMgP shows a slight indication of 1410 cm-1
carbonate band (Fig 4a) thus indicating no significant presence of CO3 group in CaMgP
powder [40] Weak band of MgHCO3 can be noticed around 1634 cm-1
[36] CaSrNaP band at
1385 cm-1
indicates incorporation of Sr2+
and Na+ ions into the structure of hydroxylapatite
(Fig 4c) [35 38] A band that indicates carbonate group (coming from the reaction medium)
in the structure of CaSrNaP is demonstrated by bands at 1417 and 1639 cm-1
[41] FTIR
spectra confirm previously described XRD results that using acetate solution multi doped
hydroxyapatite material can be obtained
FTIR spectra of thermally treated samples at 1000 ordmC for 3 h in air are shown in Fig
5 also shows presence of the characteristic signals of apatite [36] CaMgP1000 (Fig 5a)
sample shows bands in spectral regions 1470-870 cm-1
and 605-560 cm-1
which are generally
related to P-O species with different phosphorous configuration [42 43] Characteristic
vibrations of Mg2P2O7 phase are bands at 1050 605 and 560 cm-1
[42] The bands at around
1050 cm-1
belong to triply degenerate asymmetric P-O stretching mode while the band
around 955 cm-1
belongs to P-O stretching mode these bands belong to β-Ca2P2O7 phase [43]
There is no evidence of any characteristic hydroxyl group vibrations (631 and 3572 cm-1
)
CaP1000 (Fig 5b) bands are weaker than in untreated samples indicating lack of water
vibrations in structure The bands around 600 and 580 cm-1
are attributed to different types of
P-O-P bending mode vibrations which belong to β-Ca2P2O7 [43] Figure 5c shows vibration
spectrum of CaSrNaP1000 material Presence of previously described P-O and P-O-P vibrations
which belong to β-Ca2P2O7 phase also belong to Sr2P2O7 [43 44] The band around 740 cm-1
belongs to P-O-P bridge stretching vibration a striking feature of (P2O7)4-
ions in Sr2P2O7
FTIR spectrum of CaSrNaP (Fig 5c) shows no evidence of O-H stretching modes at around
3500 and 1600 cm-1
[44] Metal-oxygen vibrations in these types of systems are generally too
low to be seen in the 400-4000 cm-1
region
Fig 5 FTIR specta of thermally treated samples at 1000 degC in air for 3 h a) CaMgP1000 b)
CaP1000 and c) CaSrNaP1000
M Mirković et al Science of Sintering 50 (2018) 95-109
___________________________________________________________________________
102
Based on FTIR analysis at 1000 degC high temperature CaMgP1000 material is obtained
All of identified vibrations that belong to CaP1000 material (Fig 5b) confirm that β-Ca2P2O7 is
obtained by sintering hydroxyapatite for 3h on 1000degC in air atmosphere The analysis of
CaSrNaP1000 spectrum (Fig 5c) confirms absence of O-H vibrations and presence of (P2O7)4-
vibrations which is unique for X2P2O7 types of compounds [43 44]
313 Morphology (SEM analysis) and element composition (EDS analysis)
SEM analysis confirms that all obtained samples using precipitation method have
crystallite size of nanometer dimensions In addition SEM images (Fig 6) show that the
particle size in all samples increases with the increase of temperature Namely nanoparticles
have a natural tendency to agglomerate for two reasons firstlyminusthe agglomeration is a more
sTab configuration from an energetic point of view and secondlyminusthe agglomeration allows
the crystallite growth [45 46]
Fig 6 SEM micrographs of A) CaP CaP400 and CaP1000 B) CaMgP CaMgP400 and
CaMgP1000 C) CaSrNaP CaSrNaP400 and CaSrNaP1000 with EDS spectra
Sintering at 1000 ordmC in air for 3 h led to a higher degree of densification of
CaSrNaP1000 (98 TD) than of CaMgP1000 (86 TD) The main reason for the differences in
density may be pore and particle size distribution ie different morphology (Fig 6) The
samples after calcinations at 400 ordmC during 3 h show more uniform particles and well ordered
structure Calcinations was done in order to allow better packing and lower inner activity and
it was expected that these samples may achieve higher density after sintering This was
achieved for CaSrNaP1000 sample where the grains are uniformly sintered and the contacts
between grains show hexagonal forms and the inserted section clearly demonstrates that we
have indeed obtained dense high temperature CaSrNaP material (Fig 6c) On the other hand
CaMgP1000 (Fig 6b) exhibits grain growth due to formation of agglomerated grains
Densification retards or inhibits wide pore distribution during densification the big pores
M Mirković et alScience of Sintering 50 (2018) 95-109 ___________________________________________________________________________
103
became larger while small pores shrink and disappear [47 48] SEM micrograph of
CaMgP1000 (Fig 6b) shows numerous agglomerates of hexagonal particles with plate like
morphology bonded together with average grain size between 2 and 5 microm with non-uniform
particles with many vacancies less compact particle packing higher inner activity To
confirm the chemical composition the EDS microanalysis was done on the CaP1000
CaMgP1000 and CaSrNaP1000 sintered samples (Fig 6) It revealed that chemical composition
very similar to nominal chemical composition According to semi quantitative chemical
analysis the mean values for the ratios CaPO for CaP1000 sample determined from four runs
are 21275 These ratios correspond to empirically calculated compound with nominal
composition-(Ca21P2O75) The mean value of the chemical ratios CaMgPO for CaMgP1000
was also determined from four runs and amounts to 120727 which corresponds to
empirical compound with following composition-(Ca12Mg07)19P2O7 The mean value of the
chemical ratios Ca Sr Na PO of the NaSrHA1000 sample was determined from four runs
and amounts to 0506227 which corresponds to-Ca05Sr06Na2P2O7 These results (Fig 6)
confirm the assumption that sintered samples based on hydroxyapatite at high temperature
(1000 ordmC) remain sTab and almost nominal chemical composition The obtained EDS results
are in good agreement with XRD results
32 Electrical conductivity
Recently ionic conductors with the apatite type structures attracted a great deal of
attention as an alternative electrolytes to the conventional yttria-stabilized zirconia to operate
in ITndashSOFCs in the intermediate temperature range (600-800 ordmC) [49-52] Namely
hydroxyapatite belongs to apatitic group of phosphate minerals with general formula
Me10(XO4)6Y2 with ionic substitution ability [53] Based on XRD (Tab I) unit cell
parameters of CaMgP samples are slightly increased compared to pure CaP sample the
contraction of a and c parameter can be attributed to much smaller ionic radius of Mg2+
than
Ca2+
[54] The change of unit cell parameters a and c of CaSrNaP indicate that Sr2+
and Na+
ions are incorporated in structure and not just adhered to the crystal where Sr2+
will firstly
replace Ca2+
ion in Ca2 site [55] On the other hand Na+ ion as a smaller than Ca
2+ could
possible replace Ca2+
in Ca1 site while a vacancy may formed at the OH- site [56] At 1000degC
structural transformation of hydroxyapatite occurs and it transforms to a high temperature
mixed phosphate ceramics with M2X2O7 structure where X ions are tetrahedral coordinated
(Fig 7)
Fig 7 Structures of M2X2O7 a) αndashSr2P2O7 b) βndashCa2P2O7 and c) Na4P2O7 (the green spheres
are Sr atoms blue spheres are Ca atoms the yellow spheres are Na atoms the red spheres are
O atoms and the purple spheres are P atoms)
The crystal structures of calcium strontium and sodium phosphate materials are
comprised of diphosphate ions which are linked to metallic ions to form a three
M Mirković et al Science of Sintering 50 (2018) 95-109
___________________________________________________________________________
104
dimensional network in β ndash Ca2P2O7 α-Sr2P2O7 and Na4P2O7 structures [57 58] A
common characteristic of thermal behavior of all phosphates is that under thermal
treatment they undergo dehydratation-condensation reaction which leads to phosphate
tetrahedral bonded to mutual oxygen atom The ion conductivity in these materials
occurs mainly via O- interstitials with preferential cndashaxis conduction [50 51 56 59]
in comparison to O2-
vacancy migration in fluorite and perovskitendashbased electrolytes
[59-63] Generally the impedance spectra are presented as negative of imaginary component
of impedance (ndashZimag) versus real component of impedance (Zreal) ie as Nyquist plot The
semicircles at high and intermediate frequency are ascribed to bulk and grain boundary
processes respectively while semicircle at low frequency represents the electrode process
contribution [56] In our case for the potential application in IT-SOFCs the measurements of
ionic conductivity of electrolytes in solid state of the CaMgP1000 and CaSrNaP1000 samples
were done in intermediate temperature range of 500-700 ordmC with the increments of 50 ordmC
The original Nyquist plots recorded in the available frequency range (1 Hz-100 kHz) are
presented in Fig 8 As it could be seen (Fig 8a and 8b) with increasing the temperature the
values of both resistance elements (Rb and Rgb) obviously decrease which causes an increase
in ωmax Consequently the whole region of the impedance points shifts towards the lowndash
frequency semicircle and at higher temperatures instead of Rb and Rgb separately only the
whole sum Rb+Rgb became readable in the available frequency range In this case at higher
temperatures the time constants associated with the bulk and grain boundary impedances are
much lower than those associated with the electrode interface As a result semicircles due to
bulk and grain boundary disappear and only a single semicircle due to electrode interfacial
processes can be observed [64 66] Therefore only the whole sum Rb+Rig became readable
and the values of total resistance were estimated from the experimental cross section of
obtained semicircles with the real component of impedance (Zreal) this intercept is marked by
arrows in Fig 8 (inset) New semicircles are observed to appear in a lowndashfrequency region
being particularly visible in temperature range at 650-700 ordmC (Fig 8b) Almost doubtless it
originates from the oxygen electrode reactions O2O2-
[64] which does not belong to the
scope of this study
20 30 40 50 60 700
10
20
30
40
50
60
70 a)
20 30 40 50 60 700
1
2
3
4
5
6
7
8
9
10x103
x103
-Zim
ag
(c
m)
Zreal (cm)
x102
x102
500 C
550 C
600 C
-Zim
ag
(c
m)
Zreal (cm)
6 8 10 12 14 160
2
4
6
8
10
12
14
16 b)x102
6 8 10 12 14 160
2
x102
x102
-Zim
ag
(c
m)
Zreal (cm)
x102
650 C
700 C
-Zim
ag
(c
m)
Zreal (cm)
Fig 8 (a)(b) Complex impedance plots of the sintered samples CaSrNaP1000 measured in the
temperature range from 500 to 700 ordmC in air atmosphere The arrows indicate the points on
the real axis corresponding to the readings Rb+Rgb
The total ionic conductivity of the sintered CaSrNaP1000 sample is shown in Tab II Obtained
values (at 700 ordmC amount to 418times10-3
Ω-1
cm-1
) are slightly higher than previously published
ones [64 66] More specifically values of ionic conductivity observed at 700 ordmC in this work
M Mirković et alScience of Sintering 50 (2018) 95-109 ___________________________________________________________________________
105
are similar to values obtained at 800 and 900 ordmC [67-71] This holds also if we compare the
results we found at lower temperatures with the literature data [64-67]
Tab II The temperature dependence of total ionic conductivity (κ) of the sintered CaMgP1000
and CaSrNaP1000 sample
COMPOSITION
κ
(Ω-1
cm-1
)
500 ordmC
κ
(Ω-1
cm-1
)
550 ordmC
κ
(Ω-1
cm-1
)
600 ordmC
κ
(Ω-1
cm-1
)
650 ordmC
κ
(Ω-1
cm-1
)
700 ordmC
CaMgP1000 123times10-4
222times10-4
358times10-4
294times10-4
312times10-4
CaSrNaP1000 227times10-4
356times10-4
657times10-4
117times10-3
190times10-3
According to the results listed in Tab II the dependence log κ = f(1T) of the sintered
sample CaSrNaP1000 is presented in Fig 9 The activation energy of conductivity (Ea) was
calculated from the Arrhenius plot according to the equation
Tk
EAT a 1
lnln (1)
where σ is the conductivity T is the absolute temperature A is the pre-exponential factor and
k is the Boltzmann constant Activation energy of total conductivity for sintered CaSrNaP1000
sample is 031 eV Ea values presented in our work are much lower than previously found [67
68 70 71] In addition comparing the obtained values of Ea with activation energies of
similar oxygen ion conductors containing dopants [71] obtained Ea values were significantly
lower We suggest that this is a consequence of wellndashordered structure and better process
ability of nanopowders obtained application method
100 105 110 115 120 125 130-38
-36
-34
-32
-30
-28
-26
Ea = 031 eV
log
k
1000T (K-1)
Fig 9 The dependence log κ = f(1T) of the sintered CaSrNaP1000 sample
M Mirković et al Science of Sintering 50 (2018) 95-109
___________________________________________________________________________
106
4 Conclusions
The pure Ca and doped with Mg and NaSr hydroxyapatite (CaP CaMgP and NaSrP)
were successfully synthesized using slightly modified solution precipitation method (partial
substitution nitrates by acetate solutions) and thus we achieved higher solubility and more
cost effective synthesis Any secondary phase that formed during synthesis decomposed
during thermal treatment leaving pure CaP CaMgP and CaSrNaP hexagonal (P63m)
Ca5(PO4)3OH materials at room temperature During sintering process at 1000 ordmC in an air for
3 h we developed high temperature doped calcium phosphate materials ie CaMgP1000 and
CaSrNaP1000 materials SEM analysis showed that CaMgP1000 and CaSrNaP1000 materials have
high structural arrangement EDS analysis confirmed that we obtained phosphate compound
type-M2X2O7 The fine crystalline powders after sintering at 1000 ordmC achieved 98 of
theoretical density The maximum value of electrical conductivity at 700 ordmC was achieved for
CaSrNaP1000 material with amounts to 190times10-3
Ω-1
cm-1
Electrical characteristics of the
sintered samples indicate the superiority of solid solution for the application in SOFCs
Additionally considering high sintering temperature one can conclude that the obtained
phosphate material demonstrates high thermal stability an important characteristics in its
potential use as solid electrolyte However it is important to note that the future research
should include furthering lower temperature synthesis routes improving stability ITndashSOFCs
electrolyteelectrode molecular dynamics (MD) simulations of conduction mechanisms and
the general search for improved ion conductors Also the breadth of possible doping regimes
in this phosphate material provides new opportunities to design and optimize their transport
parameters for potential clean energy applications
Acknowledgements
This work was supported by the Ministry of Education Science and Technological
development of Serbia as part of Projects III 45012 and OI 176016
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Losilla Journal of Power Sources 245 (2014) 107
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Садржај Метода таложне титрације коришћена је за добијање чистог и допираног
хидроксиапатита јонима Sr Mg и Na (CaP CaMgP and CaSrNaP) За синтезу
супституисаних хидроксиапатитских материјала коришћена је модификована
таложна метода у односу на досада познате методе Због боље растворљивости и
знатно веће исплативости коришћени су раствори ацетата уместо нитрата
Синтетисани материјали су калцинисани на 400 degC а затим окарактерисани
рендгенском дифракцијом инфра црвеном спектроскопијом са Фуријеовом
трансформацијом и скенирајућом електронском микроскопијом Калцинисани узорци
су синтеровани на 1000 degC са задржавањем од 3 сата у атмосфери ваздуха (CaP1000
CaMgP1000 and CaSrNaP1000) Синтеровани материјали окарактерисани су рендгенском
дифракцијом на поликристалном узорку инфрацрвеном спекторскопијом са
Фуријеовом трансформацијом и скенирајућом електронском микроскопијом са
M Mirković et alScience of Sintering 50 (2018) 95-109 ___________________________________________________________________________
109
енергетско дисперзивном анализом као и методе комплексне импендансе Највећа
проводљивост добијена је на температури од 700 degC и износи 190times10-3
Ω-1
cm-1
за
мешовито допирани фосфатни материјал (CaSrNaP1000) Добијене активационе
енергије проводљивости износе 031 eV у температурном опсегу од 500-700 ordmC
Кључне речи Синтеровање Рендгенска дифракција Електрична проводљивост
Фосфатни материјали Горивне ћелије
copy 2016 Authors Published by the International Institute for the Science of Sintering This
article is an open access article distributed under the terms and conditions of the Creative
Commons mdash Attribution 40 International license
(httpscreativecommonsorglicensesby40)
M Mirković et al Science of Sintering 50 (2018) 95-109
___________________________________________________________________________
98
It is evident that addition of magnesium to the HA structure leads to the formation of high
temperature solid solution that includes high temperature CaMgP phases and hydroxyapatite
CaMgP100 nominal phase composition calculated using PDXL2 software showed 55 β-
Ca2P2O7 30 Mg2P2O7 and 15 hydroxyapatite phase
Fig 1 XRD patterns of CaPndashhydroxyapatite synthesized sample CaP400ndashhydroxyapatite
calcinated at 400 ordmC for 3 h in air and CaP1000ndashsintered at 1000 ordmC for 3 h in air
Fig 2 XRD patterns of CaMgPndashsynthesized sample CaMgP400ndashcalcined at 400 ordmC for 3 h in
air and CaMgP1000ndashsintered at 1000 ordmC for 3 h in air
The diffraction pattern of powder CaSrNaP (Fig 3) reveals that the dried precipitate exhibits
poor crystallinity peaks around 10ordm and 18deg 2θ (marked as inverted triangles) indicate
secondary phase with chemical composition Sr3(P3O9)2(H2O)7 (ICDD01-070-0007) The
secondary phase existence is common during precipitation method of nano material synthesis
[28] However there is no evidence of the existence of secondary phase in thermally treated
CaSrNaP400 (Fig 3) CaSrNaP XRD peaks are slightly shifted to the left implying a larger unit
cell most likely due to the incorporation of Sr into the HA lattice (Fig 3) Thermal treatment
of hydroxylapatite materials below 700 ordmC leads to surface reduction and single phase particle
coalescence without densification [24 29] At 1000 degC high temperature solid solution forms
noted as CaSrNaP1000 Sintering at this temperature with retention time of 3 h high
M Mirković et alScience of Sintering 50 (2018) 95-109 ___________________________________________________________________________
99
temperature calcium and strontium and sodium phosphate materials developed It is evident
that peaks belongs to three different phosphate phases β ndash Ca2P2O7 (01-071-2123) α-Sr2P2O7
(ICDD 01-072-149) and Na4P2O7 (ICDD 01-073-5982) Strontium and sodium phosphate
phases are orthorhombic while the calcium phosphate phase is tetragonal Structural changes
and phase transitions during sintering (temperatures above 700 degC) can be explained by the
loss of hydroxyl ions from the hydroxyapatite structure at temperatures around and higher
than 1000 degC and the formation of orthorhombic and tetragonal phosphate phases [24 30]
The peaks on XRD diagram are narrow and well defined indicating high structural
arrangement of sintered material
Quantitative phase analysis of CaSrNaP material showed 74 of α-Sr2P2O7 16 of β
ndash Ca2P2O7 and 10 of Na4P2O7 Hydroxylapatite is still evident at high temperatures in the
case of pure HA and HA doped with Mg On the other hand in the case high temperature
treatment (1000 degC for 3h) of multi-doped HA leads to the formation of mixed CaSrNaP
phase with high structural arrangement
Fig 3 XRD patterns of CaSrNaPndashsynthesized sample CaSrNaP400ndashcalcined at 400 ordmC for 3 h
in air and CaSrNaP1000ndashsintered at 1000 ordmC for 3 h in air
Tab I Unit cell parameters and crystallite sizes of obtained materials
Samples Cell parameter (Aring)
Crystallite size (nm) a c
CaP 938(14) 692(19) 14(13)
CaP400 939(10) 691(12) 152(5)
CaMgP 940(4) 687(5) 72(10)
CaMgP400 935(3) 688(3) 156(9)
CaSrNaP 934(4) 679(4) 43(5)
CaSrNaP400 945(6) 729(7) 79(14)
M Mirković et al Science of Sintering 50 (2018) 95-109
___________________________________________________________________________
100
Tab I shows variation of lattice parameters due to structural substitution during
precipitation and calcinations at 400 degC Unit cell parameters and crystallite size calculations
of were performed using cif files from American Mineralogist Crystal Data Structure Base
(AMCDSB)[31] (Tab I)
There is no significant difference in the average crystallite size of CaP and CaP400
samples (Tab I) Unit cell parameters of both samples are in similar to pure hydroxyapatite
(ICDD 01-071-5048) With the increase of temperature unit cell parameter a increases while
the parameter c decreases Average crystallite size of CaMgP material increasing with
temperature which is consistent with peak shapes of obtained material The unit cell
parameters of CaMgP are higher than of CaP material which is in correlation with published
data [32] Average crystallite size of thermally treated samples is slightly higher than
synthesized Factors that influence peak broadening are the presence of amorphous phase
during precipitation and a number of dislocations between crystallite boundaries [16 33]
During calcination at 400 ordmC the material becomes crystalline and thermodynamically sTab
allowing for better arrangement and growth of crystallites [34] The unit cell parameters of
CaSrNaP and CaSrNaP1000 have the highest value out of all examined materials as the ionic
radii of Sr2+
(113 Aring) and Na+ (152 Aring) are larger than ionic radii Ca
2+ (099 Aring) and Mg
2+
(069 Aring) Anisotropic change in unit cell dimensions could be attributed to the replacement of
Ca2+
with other cations ie Mg2+
Na+ and Sr
2+ in structure [35 36]
312 FTIR analysis
FTIR spectra of untreated (synthesized) samples CaP CaMgP and CaSrNaP (Fig 4)
showed the presence of characteristic apatite signals (around 563 610 657 1030 1077 cm-1
)
Fig 4b [36] The absorbed water band is around 3500 cm-1
and the band at 1413 corresponds
to CO32-
group vibration [18] In comparison with FTIR of stoichiometric HA bands of
CaSrNaP and CaMgP are red-shifted because of the presence of foreign ions incorporated into
the structure [37-39] Absence of the 631 cm-1
band that belongs to vibration modes of the
apatitic OH groups in all tree spectra most likely indicates a lack of hydroxyl content and a
possible substitution of hydroxyl groups by carbonate groups [37]
Fig 4 FTIR spectra of untreated (synthesized) samples a) CaMgP b) CaP and c) CaSrNaP
M Mirković et alScience of Sintering 50 (2018) 95-109 ___________________________________________________________________________
101
Furthermore the FTIR spectrum of CaMgP shows a slight indication of 1410 cm-1
carbonate band (Fig 4a) thus indicating no significant presence of CO3 group in CaMgP
powder [40] Weak band of MgHCO3 can be noticed around 1634 cm-1
[36] CaSrNaP band at
1385 cm-1
indicates incorporation of Sr2+
and Na+ ions into the structure of hydroxylapatite
(Fig 4c) [35 38] A band that indicates carbonate group (coming from the reaction medium)
in the structure of CaSrNaP is demonstrated by bands at 1417 and 1639 cm-1
[41] FTIR
spectra confirm previously described XRD results that using acetate solution multi doped
hydroxyapatite material can be obtained
FTIR spectra of thermally treated samples at 1000 ordmC for 3 h in air are shown in Fig
5 also shows presence of the characteristic signals of apatite [36] CaMgP1000 (Fig 5a)
sample shows bands in spectral regions 1470-870 cm-1
and 605-560 cm-1
which are generally
related to P-O species with different phosphorous configuration [42 43] Characteristic
vibrations of Mg2P2O7 phase are bands at 1050 605 and 560 cm-1
[42] The bands at around
1050 cm-1
belong to triply degenerate asymmetric P-O stretching mode while the band
around 955 cm-1
belongs to P-O stretching mode these bands belong to β-Ca2P2O7 phase [43]
There is no evidence of any characteristic hydroxyl group vibrations (631 and 3572 cm-1
)
CaP1000 (Fig 5b) bands are weaker than in untreated samples indicating lack of water
vibrations in structure The bands around 600 and 580 cm-1
are attributed to different types of
P-O-P bending mode vibrations which belong to β-Ca2P2O7 [43] Figure 5c shows vibration
spectrum of CaSrNaP1000 material Presence of previously described P-O and P-O-P vibrations
which belong to β-Ca2P2O7 phase also belong to Sr2P2O7 [43 44] The band around 740 cm-1
belongs to P-O-P bridge stretching vibration a striking feature of (P2O7)4-
ions in Sr2P2O7
FTIR spectrum of CaSrNaP (Fig 5c) shows no evidence of O-H stretching modes at around
3500 and 1600 cm-1
[44] Metal-oxygen vibrations in these types of systems are generally too
low to be seen in the 400-4000 cm-1
region
Fig 5 FTIR specta of thermally treated samples at 1000 degC in air for 3 h a) CaMgP1000 b)
CaP1000 and c) CaSrNaP1000
M Mirković et al Science of Sintering 50 (2018) 95-109
___________________________________________________________________________
102
Based on FTIR analysis at 1000 degC high temperature CaMgP1000 material is obtained
All of identified vibrations that belong to CaP1000 material (Fig 5b) confirm that β-Ca2P2O7 is
obtained by sintering hydroxyapatite for 3h on 1000degC in air atmosphere The analysis of
CaSrNaP1000 spectrum (Fig 5c) confirms absence of O-H vibrations and presence of (P2O7)4-
vibrations which is unique for X2P2O7 types of compounds [43 44]
313 Morphology (SEM analysis) and element composition (EDS analysis)
SEM analysis confirms that all obtained samples using precipitation method have
crystallite size of nanometer dimensions In addition SEM images (Fig 6) show that the
particle size in all samples increases with the increase of temperature Namely nanoparticles
have a natural tendency to agglomerate for two reasons firstlyminusthe agglomeration is a more
sTab configuration from an energetic point of view and secondlyminusthe agglomeration allows
the crystallite growth [45 46]
Fig 6 SEM micrographs of A) CaP CaP400 and CaP1000 B) CaMgP CaMgP400 and
CaMgP1000 C) CaSrNaP CaSrNaP400 and CaSrNaP1000 with EDS spectra
Sintering at 1000 ordmC in air for 3 h led to a higher degree of densification of
CaSrNaP1000 (98 TD) than of CaMgP1000 (86 TD) The main reason for the differences in
density may be pore and particle size distribution ie different morphology (Fig 6) The
samples after calcinations at 400 ordmC during 3 h show more uniform particles and well ordered
structure Calcinations was done in order to allow better packing and lower inner activity and
it was expected that these samples may achieve higher density after sintering This was
achieved for CaSrNaP1000 sample where the grains are uniformly sintered and the contacts
between grains show hexagonal forms and the inserted section clearly demonstrates that we
have indeed obtained dense high temperature CaSrNaP material (Fig 6c) On the other hand
CaMgP1000 (Fig 6b) exhibits grain growth due to formation of agglomerated grains
Densification retards or inhibits wide pore distribution during densification the big pores
M Mirković et alScience of Sintering 50 (2018) 95-109 ___________________________________________________________________________
103
became larger while small pores shrink and disappear [47 48] SEM micrograph of
CaMgP1000 (Fig 6b) shows numerous agglomerates of hexagonal particles with plate like
morphology bonded together with average grain size between 2 and 5 microm with non-uniform
particles with many vacancies less compact particle packing higher inner activity To
confirm the chemical composition the EDS microanalysis was done on the CaP1000
CaMgP1000 and CaSrNaP1000 sintered samples (Fig 6) It revealed that chemical composition
very similar to nominal chemical composition According to semi quantitative chemical
analysis the mean values for the ratios CaPO for CaP1000 sample determined from four runs
are 21275 These ratios correspond to empirically calculated compound with nominal
composition-(Ca21P2O75) The mean value of the chemical ratios CaMgPO for CaMgP1000
was also determined from four runs and amounts to 120727 which corresponds to
empirical compound with following composition-(Ca12Mg07)19P2O7 The mean value of the
chemical ratios Ca Sr Na PO of the NaSrHA1000 sample was determined from four runs
and amounts to 0506227 which corresponds to-Ca05Sr06Na2P2O7 These results (Fig 6)
confirm the assumption that sintered samples based on hydroxyapatite at high temperature
(1000 ordmC) remain sTab and almost nominal chemical composition The obtained EDS results
are in good agreement with XRD results
32 Electrical conductivity
Recently ionic conductors with the apatite type structures attracted a great deal of
attention as an alternative electrolytes to the conventional yttria-stabilized zirconia to operate
in ITndashSOFCs in the intermediate temperature range (600-800 ordmC) [49-52] Namely
hydroxyapatite belongs to apatitic group of phosphate minerals with general formula
Me10(XO4)6Y2 with ionic substitution ability [53] Based on XRD (Tab I) unit cell
parameters of CaMgP samples are slightly increased compared to pure CaP sample the
contraction of a and c parameter can be attributed to much smaller ionic radius of Mg2+
than
Ca2+
[54] The change of unit cell parameters a and c of CaSrNaP indicate that Sr2+
and Na+
ions are incorporated in structure and not just adhered to the crystal where Sr2+
will firstly
replace Ca2+
ion in Ca2 site [55] On the other hand Na+ ion as a smaller than Ca
2+ could
possible replace Ca2+
in Ca1 site while a vacancy may formed at the OH- site [56] At 1000degC
structural transformation of hydroxyapatite occurs and it transforms to a high temperature
mixed phosphate ceramics with M2X2O7 structure where X ions are tetrahedral coordinated
(Fig 7)
Fig 7 Structures of M2X2O7 a) αndashSr2P2O7 b) βndashCa2P2O7 and c) Na4P2O7 (the green spheres
are Sr atoms blue spheres are Ca atoms the yellow spheres are Na atoms the red spheres are
O atoms and the purple spheres are P atoms)
The crystal structures of calcium strontium and sodium phosphate materials are
comprised of diphosphate ions which are linked to metallic ions to form a three
M Mirković et al Science of Sintering 50 (2018) 95-109
___________________________________________________________________________
104
dimensional network in β ndash Ca2P2O7 α-Sr2P2O7 and Na4P2O7 structures [57 58] A
common characteristic of thermal behavior of all phosphates is that under thermal
treatment they undergo dehydratation-condensation reaction which leads to phosphate
tetrahedral bonded to mutual oxygen atom The ion conductivity in these materials
occurs mainly via O- interstitials with preferential cndashaxis conduction [50 51 56 59]
in comparison to O2-
vacancy migration in fluorite and perovskitendashbased electrolytes
[59-63] Generally the impedance spectra are presented as negative of imaginary component
of impedance (ndashZimag) versus real component of impedance (Zreal) ie as Nyquist plot The
semicircles at high and intermediate frequency are ascribed to bulk and grain boundary
processes respectively while semicircle at low frequency represents the electrode process
contribution [56] In our case for the potential application in IT-SOFCs the measurements of
ionic conductivity of electrolytes in solid state of the CaMgP1000 and CaSrNaP1000 samples
were done in intermediate temperature range of 500-700 ordmC with the increments of 50 ordmC
The original Nyquist plots recorded in the available frequency range (1 Hz-100 kHz) are
presented in Fig 8 As it could be seen (Fig 8a and 8b) with increasing the temperature the
values of both resistance elements (Rb and Rgb) obviously decrease which causes an increase
in ωmax Consequently the whole region of the impedance points shifts towards the lowndash
frequency semicircle and at higher temperatures instead of Rb and Rgb separately only the
whole sum Rb+Rgb became readable in the available frequency range In this case at higher
temperatures the time constants associated with the bulk and grain boundary impedances are
much lower than those associated with the electrode interface As a result semicircles due to
bulk and grain boundary disappear and only a single semicircle due to electrode interfacial
processes can be observed [64 66] Therefore only the whole sum Rb+Rig became readable
and the values of total resistance were estimated from the experimental cross section of
obtained semicircles with the real component of impedance (Zreal) this intercept is marked by
arrows in Fig 8 (inset) New semicircles are observed to appear in a lowndashfrequency region
being particularly visible in temperature range at 650-700 ordmC (Fig 8b) Almost doubtless it
originates from the oxygen electrode reactions O2O2-
[64] which does not belong to the
scope of this study
20 30 40 50 60 700
10
20
30
40
50
60
70 a)
20 30 40 50 60 700
1
2
3
4
5
6
7
8
9
10x103
x103
-Zim
ag
(c
m)
Zreal (cm)
x102
x102
500 C
550 C
600 C
-Zim
ag
(c
m)
Zreal (cm)
6 8 10 12 14 160
2
4
6
8
10
12
14
16 b)x102
6 8 10 12 14 160
2
x102
x102
-Zim
ag
(c
m)
Zreal (cm)
x102
650 C
700 C
-Zim
ag
(c
m)
Zreal (cm)
Fig 8 (a)(b) Complex impedance plots of the sintered samples CaSrNaP1000 measured in the
temperature range from 500 to 700 ordmC in air atmosphere The arrows indicate the points on
the real axis corresponding to the readings Rb+Rgb
The total ionic conductivity of the sintered CaSrNaP1000 sample is shown in Tab II Obtained
values (at 700 ordmC amount to 418times10-3
Ω-1
cm-1
) are slightly higher than previously published
ones [64 66] More specifically values of ionic conductivity observed at 700 ordmC in this work
M Mirković et alScience of Sintering 50 (2018) 95-109 ___________________________________________________________________________
105
are similar to values obtained at 800 and 900 ordmC [67-71] This holds also if we compare the
results we found at lower temperatures with the literature data [64-67]
Tab II The temperature dependence of total ionic conductivity (κ) of the sintered CaMgP1000
and CaSrNaP1000 sample
COMPOSITION
κ
(Ω-1
cm-1
)
500 ordmC
κ
(Ω-1
cm-1
)
550 ordmC
κ
(Ω-1
cm-1
)
600 ordmC
κ
(Ω-1
cm-1
)
650 ordmC
κ
(Ω-1
cm-1
)
700 ordmC
CaMgP1000 123times10-4
222times10-4
358times10-4
294times10-4
312times10-4
CaSrNaP1000 227times10-4
356times10-4
657times10-4
117times10-3
190times10-3
According to the results listed in Tab II the dependence log κ = f(1T) of the sintered
sample CaSrNaP1000 is presented in Fig 9 The activation energy of conductivity (Ea) was
calculated from the Arrhenius plot according to the equation
Tk
EAT a 1
lnln (1)
where σ is the conductivity T is the absolute temperature A is the pre-exponential factor and
k is the Boltzmann constant Activation energy of total conductivity for sintered CaSrNaP1000
sample is 031 eV Ea values presented in our work are much lower than previously found [67
68 70 71] In addition comparing the obtained values of Ea with activation energies of
similar oxygen ion conductors containing dopants [71] obtained Ea values were significantly
lower We suggest that this is a consequence of wellndashordered structure and better process
ability of nanopowders obtained application method
100 105 110 115 120 125 130-38
-36
-34
-32
-30
-28
-26
Ea = 031 eV
log
k
1000T (K-1)
Fig 9 The dependence log κ = f(1T) of the sintered CaSrNaP1000 sample
M Mirković et al Science of Sintering 50 (2018) 95-109
___________________________________________________________________________
106
4 Conclusions
The pure Ca and doped with Mg and NaSr hydroxyapatite (CaP CaMgP and NaSrP)
were successfully synthesized using slightly modified solution precipitation method (partial
substitution nitrates by acetate solutions) and thus we achieved higher solubility and more
cost effective synthesis Any secondary phase that formed during synthesis decomposed
during thermal treatment leaving pure CaP CaMgP and CaSrNaP hexagonal (P63m)
Ca5(PO4)3OH materials at room temperature During sintering process at 1000 ordmC in an air for
3 h we developed high temperature doped calcium phosphate materials ie CaMgP1000 and
CaSrNaP1000 materials SEM analysis showed that CaMgP1000 and CaSrNaP1000 materials have
high structural arrangement EDS analysis confirmed that we obtained phosphate compound
type-M2X2O7 The fine crystalline powders after sintering at 1000 ordmC achieved 98 of
theoretical density The maximum value of electrical conductivity at 700 ordmC was achieved for
CaSrNaP1000 material with amounts to 190times10-3
Ω-1
cm-1
Electrical characteristics of the
sintered samples indicate the superiority of solid solution for the application in SOFCs
Additionally considering high sintering temperature one can conclude that the obtained
phosphate material demonstrates high thermal stability an important characteristics in its
potential use as solid electrolyte However it is important to note that the future research
should include furthering lower temperature synthesis routes improving stability ITndashSOFCs
electrolyteelectrode molecular dynamics (MD) simulations of conduction mechanisms and
the general search for improved ion conductors Also the breadth of possible doping regimes
in this phosphate material provides new opportunities to design and optimize their transport
parameters for potential clean energy applications
Acknowledgements
This work was supported by the Ministry of Education Science and Technological
development of Serbia as part of Projects III 45012 and OI 176016
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Садржај Метода таложне титрације коришћена је за добијање чистог и допираног
хидроксиапатита јонима Sr Mg и Na (CaP CaMgP and CaSrNaP) За синтезу
супституисаних хидроксиапатитских материјала коришћена је модификована
таложна метода у односу на досада познате методе Због боље растворљивости и
знатно веће исплативости коришћени су раствори ацетата уместо нитрата
Синтетисани материјали су калцинисани на 400 degC а затим окарактерисани
рендгенском дифракцијом инфра црвеном спектроскопијом са Фуријеовом
трансформацијом и скенирајућом електронском микроскопијом Калцинисани узорци
су синтеровани на 1000 degC са задржавањем од 3 сата у атмосфери ваздуха (CaP1000
CaMgP1000 and CaSrNaP1000) Синтеровани материјали окарактерисани су рендгенском
дифракцијом на поликристалном узорку инфрацрвеном спекторскопијом са
Фуријеовом трансформацијом и скенирајућом електронском микроскопијом са
M Mirković et alScience of Sintering 50 (2018) 95-109 ___________________________________________________________________________
109
енергетско дисперзивном анализом као и методе комплексне импендансе Највећа
проводљивост добијена је на температури од 700 degC и износи 190times10-3
Ω-1
cm-1
за
мешовито допирани фосфатни материјал (CaSrNaP1000) Добијене активационе
енергије проводљивости износе 031 eV у температурном опсегу од 500-700 ordmC
Кључне речи Синтеровање Рендгенска дифракција Електрична проводљивост
Фосфатни материјали Горивне ћелије
copy 2016 Authors Published by the International Institute for the Science of Sintering This
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M Mirković et alScience of Sintering 50 (2018) 95-109 ___________________________________________________________________________
99
temperature calcium and strontium and sodium phosphate materials developed It is evident
that peaks belongs to three different phosphate phases β ndash Ca2P2O7 (01-071-2123) α-Sr2P2O7
(ICDD 01-072-149) and Na4P2O7 (ICDD 01-073-5982) Strontium and sodium phosphate
phases are orthorhombic while the calcium phosphate phase is tetragonal Structural changes
and phase transitions during sintering (temperatures above 700 degC) can be explained by the
loss of hydroxyl ions from the hydroxyapatite structure at temperatures around and higher
than 1000 degC and the formation of orthorhombic and tetragonal phosphate phases [24 30]
The peaks on XRD diagram are narrow and well defined indicating high structural
arrangement of sintered material
Quantitative phase analysis of CaSrNaP material showed 74 of α-Sr2P2O7 16 of β
ndash Ca2P2O7 and 10 of Na4P2O7 Hydroxylapatite is still evident at high temperatures in the
case of pure HA and HA doped with Mg On the other hand in the case high temperature
treatment (1000 degC for 3h) of multi-doped HA leads to the formation of mixed CaSrNaP
phase with high structural arrangement
Fig 3 XRD patterns of CaSrNaPndashsynthesized sample CaSrNaP400ndashcalcined at 400 ordmC for 3 h
in air and CaSrNaP1000ndashsintered at 1000 ordmC for 3 h in air
Tab I Unit cell parameters and crystallite sizes of obtained materials
Samples Cell parameter (Aring)
Crystallite size (nm) a c
CaP 938(14) 692(19) 14(13)
CaP400 939(10) 691(12) 152(5)
CaMgP 940(4) 687(5) 72(10)
CaMgP400 935(3) 688(3) 156(9)
CaSrNaP 934(4) 679(4) 43(5)
CaSrNaP400 945(6) 729(7) 79(14)
M Mirković et al Science of Sintering 50 (2018) 95-109
___________________________________________________________________________
100
Tab I shows variation of lattice parameters due to structural substitution during
precipitation and calcinations at 400 degC Unit cell parameters and crystallite size calculations
of were performed using cif files from American Mineralogist Crystal Data Structure Base
(AMCDSB)[31] (Tab I)
There is no significant difference in the average crystallite size of CaP and CaP400
samples (Tab I) Unit cell parameters of both samples are in similar to pure hydroxyapatite
(ICDD 01-071-5048) With the increase of temperature unit cell parameter a increases while
the parameter c decreases Average crystallite size of CaMgP material increasing with
temperature which is consistent with peak shapes of obtained material The unit cell
parameters of CaMgP are higher than of CaP material which is in correlation with published
data [32] Average crystallite size of thermally treated samples is slightly higher than
synthesized Factors that influence peak broadening are the presence of amorphous phase
during precipitation and a number of dislocations between crystallite boundaries [16 33]
During calcination at 400 ordmC the material becomes crystalline and thermodynamically sTab
allowing for better arrangement and growth of crystallites [34] The unit cell parameters of
CaSrNaP and CaSrNaP1000 have the highest value out of all examined materials as the ionic
radii of Sr2+
(113 Aring) and Na+ (152 Aring) are larger than ionic radii Ca
2+ (099 Aring) and Mg
2+
(069 Aring) Anisotropic change in unit cell dimensions could be attributed to the replacement of
Ca2+
with other cations ie Mg2+
Na+ and Sr
2+ in structure [35 36]
312 FTIR analysis
FTIR spectra of untreated (synthesized) samples CaP CaMgP and CaSrNaP (Fig 4)
showed the presence of characteristic apatite signals (around 563 610 657 1030 1077 cm-1
)
Fig 4b [36] The absorbed water band is around 3500 cm-1
and the band at 1413 corresponds
to CO32-
group vibration [18] In comparison with FTIR of stoichiometric HA bands of
CaSrNaP and CaMgP are red-shifted because of the presence of foreign ions incorporated into
the structure [37-39] Absence of the 631 cm-1
band that belongs to vibration modes of the
apatitic OH groups in all tree spectra most likely indicates a lack of hydroxyl content and a
possible substitution of hydroxyl groups by carbonate groups [37]
Fig 4 FTIR spectra of untreated (synthesized) samples a) CaMgP b) CaP and c) CaSrNaP
M Mirković et alScience of Sintering 50 (2018) 95-109 ___________________________________________________________________________
101
Furthermore the FTIR spectrum of CaMgP shows a slight indication of 1410 cm-1
carbonate band (Fig 4a) thus indicating no significant presence of CO3 group in CaMgP
powder [40] Weak band of MgHCO3 can be noticed around 1634 cm-1
[36] CaSrNaP band at
1385 cm-1
indicates incorporation of Sr2+
and Na+ ions into the structure of hydroxylapatite
(Fig 4c) [35 38] A band that indicates carbonate group (coming from the reaction medium)
in the structure of CaSrNaP is demonstrated by bands at 1417 and 1639 cm-1
[41] FTIR
spectra confirm previously described XRD results that using acetate solution multi doped
hydroxyapatite material can be obtained
FTIR spectra of thermally treated samples at 1000 ordmC for 3 h in air are shown in Fig
5 also shows presence of the characteristic signals of apatite [36] CaMgP1000 (Fig 5a)
sample shows bands in spectral regions 1470-870 cm-1
and 605-560 cm-1
which are generally
related to P-O species with different phosphorous configuration [42 43] Characteristic
vibrations of Mg2P2O7 phase are bands at 1050 605 and 560 cm-1
[42] The bands at around
1050 cm-1
belong to triply degenerate asymmetric P-O stretching mode while the band
around 955 cm-1
belongs to P-O stretching mode these bands belong to β-Ca2P2O7 phase [43]
There is no evidence of any characteristic hydroxyl group vibrations (631 and 3572 cm-1
)
CaP1000 (Fig 5b) bands are weaker than in untreated samples indicating lack of water
vibrations in structure The bands around 600 and 580 cm-1
are attributed to different types of
P-O-P bending mode vibrations which belong to β-Ca2P2O7 [43] Figure 5c shows vibration
spectrum of CaSrNaP1000 material Presence of previously described P-O and P-O-P vibrations
which belong to β-Ca2P2O7 phase also belong to Sr2P2O7 [43 44] The band around 740 cm-1
belongs to P-O-P bridge stretching vibration a striking feature of (P2O7)4-
ions in Sr2P2O7
FTIR spectrum of CaSrNaP (Fig 5c) shows no evidence of O-H stretching modes at around
3500 and 1600 cm-1
[44] Metal-oxygen vibrations in these types of systems are generally too
low to be seen in the 400-4000 cm-1
region
Fig 5 FTIR specta of thermally treated samples at 1000 degC in air for 3 h a) CaMgP1000 b)
CaP1000 and c) CaSrNaP1000
M Mirković et al Science of Sintering 50 (2018) 95-109
___________________________________________________________________________
102
Based on FTIR analysis at 1000 degC high temperature CaMgP1000 material is obtained
All of identified vibrations that belong to CaP1000 material (Fig 5b) confirm that β-Ca2P2O7 is
obtained by sintering hydroxyapatite for 3h on 1000degC in air atmosphere The analysis of
CaSrNaP1000 spectrum (Fig 5c) confirms absence of O-H vibrations and presence of (P2O7)4-
vibrations which is unique for X2P2O7 types of compounds [43 44]
313 Morphology (SEM analysis) and element composition (EDS analysis)
SEM analysis confirms that all obtained samples using precipitation method have
crystallite size of nanometer dimensions In addition SEM images (Fig 6) show that the
particle size in all samples increases with the increase of temperature Namely nanoparticles
have a natural tendency to agglomerate for two reasons firstlyminusthe agglomeration is a more
sTab configuration from an energetic point of view and secondlyminusthe agglomeration allows
the crystallite growth [45 46]
Fig 6 SEM micrographs of A) CaP CaP400 and CaP1000 B) CaMgP CaMgP400 and
CaMgP1000 C) CaSrNaP CaSrNaP400 and CaSrNaP1000 with EDS spectra
Sintering at 1000 ordmC in air for 3 h led to a higher degree of densification of
CaSrNaP1000 (98 TD) than of CaMgP1000 (86 TD) The main reason for the differences in
density may be pore and particle size distribution ie different morphology (Fig 6) The
samples after calcinations at 400 ordmC during 3 h show more uniform particles and well ordered
structure Calcinations was done in order to allow better packing and lower inner activity and
it was expected that these samples may achieve higher density after sintering This was
achieved for CaSrNaP1000 sample where the grains are uniformly sintered and the contacts
between grains show hexagonal forms and the inserted section clearly demonstrates that we
have indeed obtained dense high temperature CaSrNaP material (Fig 6c) On the other hand
CaMgP1000 (Fig 6b) exhibits grain growth due to formation of agglomerated grains
Densification retards or inhibits wide pore distribution during densification the big pores
M Mirković et alScience of Sintering 50 (2018) 95-109 ___________________________________________________________________________
103
became larger while small pores shrink and disappear [47 48] SEM micrograph of
CaMgP1000 (Fig 6b) shows numerous agglomerates of hexagonal particles with plate like
morphology bonded together with average grain size between 2 and 5 microm with non-uniform
particles with many vacancies less compact particle packing higher inner activity To
confirm the chemical composition the EDS microanalysis was done on the CaP1000
CaMgP1000 and CaSrNaP1000 sintered samples (Fig 6) It revealed that chemical composition
very similar to nominal chemical composition According to semi quantitative chemical
analysis the mean values for the ratios CaPO for CaP1000 sample determined from four runs
are 21275 These ratios correspond to empirically calculated compound with nominal
composition-(Ca21P2O75) The mean value of the chemical ratios CaMgPO for CaMgP1000
was also determined from four runs and amounts to 120727 which corresponds to
empirical compound with following composition-(Ca12Mg07)19P2O7 The mean value of the
chemical ratios Ca Sr Na PO of the NaSrHA1000 sample was determined from four runs
and amounts to 0506227 which corresponds to-Ca05Sr06Na2P2O7 These results (Fig 6)
confirm the assumption that sintered samples based on hydroxyapatite at high temperature
(1000 ordmC) remain sTab and almost nominal chemical composition The obtained EDS results
are in good agreement with XRD results
32 Electrical conductivity
Recently ionic conductors with the apatite type structures attracted a great deal of
attention as an alternative electrolytes to the conventional yttria-stabilized zirconia to operate
in ITndashSOFCs in the intermediate temperature range (600-800 ordmC) [49-52] Namely
hydroxyapatite belongs to apatitic group of phosphate minerals with general formula
Me10(XO4)6Y2 with ionic substitution ability [53] Based on XRD (Tab I) unit cell
parameters of CaMgP samples are slightly increased compared to pure CaP sample the
contraction of a and c parameter can be attributed to much smaller ionic radius of Mg2+
than
Ca2+
[54] The change of unit cell parameters a and c of CaSrNaP indicate that Sr2+
and Na+
ions are incorporated in structure and not just adhered to the crystal where Sr2+
will firstly
replace Ca2+
ion in Ca2 site [55] On the other hand Na+ ion as a smaller than Ca
2+ could
possible replace Ca2+
in Ca1 site while a vacancy may formed at the OH- site [56] At 1000degC
structural transformation of hydroxyapatite occurs and it transforms to a high temperature
mixed phosphate ceramics with M2X2O7 structure where X ions are tetrahedral coordinated
(Fig 7)
Fig 7 Structures of M2X2O7 a) αndashSr2P2O7 b) βndashCa2P2O7 and c) Na4P2O7 (the green spheres
are Sr atoms blue spheres are Ca atoms the yellow spheres are Na atoms the red spheres are
O atoms and the purple spheres are P atoms)
The crystal structures of calcium strontium and sodium phosphate materials are
comprised of diphosphate ions which are linked to metallic ions to form a three
M Mirković et al Science of Sintering 50 (2018) 95-109
___________________________________________________________________________
104
dimensional network in β ndash Ca2P2O7 α-Sr2P2O7 and Na4P2O7 structures [57 58] A
common characteristic of thermal behavior of all phosphates is that under thermal
treatment they undergo dehydratation-condensation reaction which leads to phosphate
tetrahedral bonded to mutual oxygen atom The ion conductivity in these materials
occurs mainly via O- interstitials with preferential cndashaxis conduction [50 51 56 59]
in comparison to O2-
vacancy migration in fluorite and perovskitendashbased electrolytes
[59-63] Generally the impedance spectra are presented as negative of imaginary component
of impedance (ndashZimag) versus real component of impedance (Zreal) ie as Nyquist plot The
semicircles at high and intermediate frequency are ascribed to bulk and grain boundary
processes respectively while semicircle at low frequency represents the electrode process
contribution [56] In our case for the potential application in IT-SOFCs the measurements of
ionic conductivity of electrolytes in solid state of the CaMgP1000 and CaSrNaP1000 samples
were done in intermediate temperature range of 500-700 ordmC with the increments of 50 ordmC
The original Nyquist plots recorded in the available frequency range (1 Hz-100 kHz) are
presented in Fig 8 As it could be seen (Fig 8a and 8b) with increasing the temperature the
values of both resistance elements (Rb and Rgb) obviously decrease which causes an increase
in ωmax Consequently the whole region of the impedance points shifts towards the lowndash
frequency semicircle and at higher temperatures instead of Rb and Rgb separately only the
whole sum Rb+Rgb became readable in the available frequency range In this case at higher
temperatures the time constants associated with the bulk and grain boundary impedances are
much lower than those associated with the electrode interface As a result semicircles due to
bulk and grain boundary disappear and only a single semicircle due to electrode interfacial
processes can be observed [64 66] Therefore only the whole sum Rb+Rig became readable
and the values of total resistance were estimated from the experimental cross section of
obtained semicircles with the real component of impedance (Zreal) this intercept is marked by
arrows in Fig 8 (inset) New semicircles are observed to appear in a lowndashfrequency region
being particularly visible in temperature range at 650-700 ordmC (Fig 8b) Almost doubtless it
originates from the oxygen electrode reactions O2O2-
[64] which does not belong to the
scope of this study
20 30 40 50 60 700
10
20
30
40
50
60
70 a)
20 30 40 50 60 700
1
2
3
4
5
6
7
8
9
10x103
x103
-Zim
ag
(c
m)
Zreal (cm)
x102
x102
500 C
550 C
600 C
-Zim
ag
(c
m)
Zreal (cm)
6 8 10 12 14 160
2
4
6
8
10
12
14
16 b)x102
6 8 10 12 14 160
2
x102
x102
-Zim
ag
(c
m)
Zreal (cm)
x102
650 C
700 C
-Zim
ag
(c
m)
Zreal (cm)
Fig 8 (a)(b) Complex impedance plots of the sintered samples CaSrNaP1000 measured in the
temperature range from 500 to 700 ordmC in air atmosphere The arrows indicate the points on
the real axis corresponding to the readings Rb+Rgb
The total ionic conductivity of the sintered CaSrNaP1000 sample is shown in Tab II Obtained
values (at 700 ordmC amount to 418times10-3
Ω-1
cm-1
) are slightly higher than previously published
ones [64 66] More specifically values of ionic conductivity observed at 700 ordmC in this work
M Mirković et alScience of Sintering 50 (2018) 95-109 ___________________________________________________________________________
105
are similar to values obtained at 800 and 900 ordmC [67-71] This holds also if we compare the
results we found at lower temperatures with the literature data [64-67]
Tab II The temperature dependence of total ionic conductivity (κ) of the sintered CaMgP1000
and CaSrNaP1000 sample
COMPOSITION
κ
(Ω-1
cm-1
)
500 ordmC
κ
(Ω-1
cm-1
)
550 ordmC
κ
(Ω-1
cm-1
)
600 ordmC
κ
(Ω-1
cm-1
)
650 ordmC
κ
(Ω-1
cm-1
)
700 ordmC
CaMgP1000 123times10-4
222times10-4
358times10-4
294times10-4
312times10-4
CaSrNaP1000 227times10-4
356times10-4
657times10-4
117times10-3
190times10-3
According to the results listed in Tab II the dependence log κ = f(1T) of the sintered
sample CaSrNaP1000 is presented in Fig 9 The activation energy of conductivity (Ea) was
calculated from the Arrhenius plot according to the equation
Tk
EAT a 1
lnln (1)
where σ is the conductivity T is the absolute temperature A is the pre-exponential factor and
k is the Boltzmann constant Activation energy of total conductivity for sintered CaSrNaP1000
sample is 031 eV Ea values presented in our work are much lower than previously found [67
68 70 71] In addition comparing the obtained values of Ea with activation energies of
similar oxygen ion conductors containing dopants [71] obtained Ea values were significantly
lower We suggest that this is a consequence of wellndashordered structure and better process
ability of nanopowders obtained application method
100 105 110 115 120 125 130-38
-36
-34
-32
-30
-28
-26
Ea = 031 eV
log
k
1000T (K-1)
Fig 9 The dependence log κ = f(1T) of the sintered CaSrNaP1000 sample
M Mirković et al Science of Sintering 50 (2018) 95-109
___________________________________________________________________________
106
4 Conclusions
The pure Ca and doped with Mg and NaSr hydroxyapatite (CaP CaMgP and NaSrP)
were successfully synthesized using slightly modified solution precipitation method (partial
substitution nitrates by acetate solutions) and thus we achieved higher solubility and more
cost effective synthesis Any secondary phase that formed during synthesis decomposed
during thermal treatment leaving pure CaP CaMgP and CaSrNaP hexagonal (P63m)
Ca5(PO4)3OH materials at room temperature During sintering process at 1000 ordmC in an air for
3 h we developed high temperature doped calcium phosphate materials ie CaMgP1000 and
CaSrNaP1000 materials SEM analysis showed that CaMgP1000 and CaSrNaP1000 materials have
high structural arrangement EDS analysis confirmed that we obtained phosphate compound
type-M2X2O7 The fine crystalline powders after sintering at 1000 ordmC achieved 98 of
theoretical density The maximum value of electrical conductivity at 700 ordmC was achieved for
CaSrNaP1000 material with amounts to 190times10-3
Ω-1
cm-1
Electrical characteristics of the
sintered samples indicate the superiority of solid solution for the application in SOFCs
Additionally considering high sintering temperature one can conclude that the obtained
phosphate material demonstrates high thermal stability an important characteristics in its
potential use as solid electrolyte However it is important to note that the future research
should include furthering lower temperature synthesis routes improving stability ITndashSOFCs
electrolyteelectrode molecular dynamics (MD) simulations of conduction mechanisms and
the general search for improved ion conductors Also the breadth of possible doping regimes
in this phosphate material provides new opportunities to design and optimize their transport
parameters for potential clean energy applications
Acknowledgements
This work was supported by the Ministry of Education Science and Technological
development of Serbia as part of Projects III 45012 and OI 176016
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Садржај Метода таложне титрације коришћена је за добијање чистог и допираног
хидроксиапатита јонима Sr Mg и Na (CaP CaMgP and CaSrNaP) За синтезу
супституисаних хидроксиапатитских материјала коришћена је модификована
таложна метода у односу на досада познате методе Због боље растворљивости и
знатно веће исплативости коришћени су раствори ацетата уместо нитрата
Синтетисани материјали су калцинисани на 400 degC а затим окарактерисани
рендгенском дифракцијом инфра црвеном спектроскопијом са Фуријеовом
трансформацијом и скенирајућом електронском микроскопијом Калцинисани узорци
су синтеровани на 1000 degC са задржавањем од 3 сата у атмосфери ваздуха (CaP1000
CaMgP1000 and CaSrNaP1000) Синтеровани материјали окарактерисани су рендгенском
дифракцијом на поликристалном узорку инфрацрвеном спекторскопијом са
Фуријеовом трансформацијом и скенирајућом електронском микроскопијом са
M Mirković et alScience of Sintering 50 (2018) 95-109 ___________________________________________________________________________
109
енергетско дисперзивном анализом као и методе комплексне импендансе Највећа
проводљивост добијена је на температури од 700 degC и износи 190times10-3
Ω-1
cm-1
за
мешовито допирани фосфатни материјал (CaSrNaP1000) Добијене активационе
енергије проводљивости износе 031 eV у температурном опсегу од 500-700 ordmC
Кључне речи Синтеровање Рендгенска дифракција Електрична проводљивост
Фосфатни материјали Горивне ћелије
copy 2016 Authors Published by the International Institute for the Science of Sintering This
article is an open access article distributed under the terms and conditions of the Creative
Commons mdash Attribution 40 International license
(httpscreativecommonsorglicensesby40)
M Mirković et al Science of Sintering 50 (2018) 95-109
___________________________________________________________________________
100
Tab I shows variation of lattice parameters due to structural substitution during
precipitation and calcinations at 400 degC Unit cell parameters and crystallite size calculations
of were performed using cif files from American Mineralogist Crystal Data Structure Base
(AMCDSB)[31] (Tab I)
There is no significant difference in the average crystallite size of CaP and CaP400
samples (Tab I) Unit cell parameters of both samples are in similar to pure hydroxyapatite
(ICDD 01-071-5048) With the increase of temperature unit cell parameter a increases while
the parameter c decreases Average crystallite size of CaMgP material increasing with
temperature which is consistent with peak shapes of obtained material The unit cell
parameters of CaMgP are higher than of CaP material which is in correlation with published
data [32] Average crystallite size of thermally treated samples is slightly higher than
synthesized Factors that influence peak broadening are the presence of amorphous phase
during precipitation and a number of dislocations between crystallite boundaries [16 33]
During calcination at 400 ordmC the material becomes crystalline and thermodynamically sTab
allowing for better arrangement and growth of crystallites [34] The unit cell parameters of
CaSrNaP and CaSrNaP1000 have the highest value out of all examined materials as the ionic
radii of Sr2+
(113 Aring) and Na+ (152 Aring) are larger than ionic radii Ca
2+ (099 Aring) and Mg
2+
(069 Aring) Anisotropic change in unit cell dimensions could be attributed to the replacement of
Ca2+
with other cations ie Mg2+
Na+ and Sr
2+ in structure [35 36]
312 FTIR analysis
FTIR spectra of untreated (synthesized) samples CaP CaMgP and CaSrNaP (Fig 4)
showed the presence of characteristic apatite signals (around 563 610 657 1030 1077 cm-1
)
Fig 4b [36] The absorbed water band is around 3500 cm-1
and the band at 1413 corresponds
to CO32-
group vibration [18] In comparison with FTIR of stoichiometric HA bands of
CaSrNaP and CaMgP are red-shifted because of the presence of foreign ions incorporated into
the structure [37-39] Absence of the 631 cm-1
band that belongs to vibration modes of the
apatitic OH groups in all tree spectra most likely indicates a lack of hydroxyl content and a
possible substitution of hydroxyl groups by carbonate groups [37]
Fig 4 FTIR spectra of untreated (synthesized) samples a) CaMgP b) CaP and c) CaSrNaP
M Mirković et alScience of Sintering 50 (2018) 95-109 ___________________________________________________________________________
101
Furthermore the FTIR spectrum of CaMgP shows a slight indication of 1410 cm-1
carbonate band (Fig 4a) thus indicating no significant presence of CO3 group in CaMgP
powder [40] Weak band of MgHCO3 can be noticed around 1634 cm-1
[36] CaSrNaP band at
1385 cm-1
indicates incorporation of Sr2+
and Na+ ions into the structure of hydroxylapatite
(Fig 4c) [35 38] A band that indicates carbonate group (coming from the reaction medium)
in the structure of CaSrNaP is demonstrated by bands at 1417 and 1639 cm-1
[41] FTIR
spectra confirm previously described XRD results that using acetate solution multi doped
hydroxyapatite material can be obtained
FTIR spectra of thermally treated samples at 1000 ordmC for 3 h in air are shown in Fig
5 also shows presence of the characteristic signals of apatite [36] CaMgP1000 (Fig 5a)
sample shows bands in spectral regions 1470-870 cm-1
and 605-560 cm-1
which are generally
related to P-O species with different phosphorous configuration [42 43] Characteristic
vibrations of Mg2P2O7 phase are bands at 1050 605 and 560 cm-1
[42] The bands at around
1050 cm-1
belong to triply degenerate asymmetric P-O stretching mode while the band
around 955 cm-1
belongs to P-O stretching mode these bands belong to β-Ca2P2O7 phase [43]
There is no evidence of any characteristic hydroxyl group vibrations (631 and 3572 cm-1
)
CaP1000 (Fig 5b) bands are weaker than in untreated samples indicating lack of water
vibrations in structure The bands around 600 and 580 cm-1
are attributed to different types of
P-O-P bending mode vibrations which belong to β-Ca2P2O7 [43] Figure 5c shows vibration
spectrum of CaSrNaP1000 material Presence of previously described P-O and P-O-P vibrations
which belong to β-Ca2P2O7 phase also belong to Sr2P2O7 [43 44] The band around 740 cm-1
belongs to P-O-P bridge stretching vibration a striking feature of (P2O7)4-
ions in Sr2P2O7
FTIR spectrum of CaSrNaP (Fig 5c) shows no evidence of O-H stretching modes at around
3500 and 1600 cm-1
[44] Metal-oxygen vibrations in these types of systems are generally too
low to be seen in the 400-4000 cm-1
region
Fig 5 FTIR specta of thermally treated samples at 1000 degC in air for 3 h a) CaMgP1000 b)
CaP1000 and c) CaSrNaP1000
M Mirković et al Science of Sintering 50 (2018) 95-109
___________________________________________________________________________
102
Based on FTIR analysis at 1000 degC high temperature CaMgP1000 material is obtained
All of identified vibrations that belong to CaP1000 material (Fig 5b) confirm that β-Ca2P2O7 is
obtained by sintering hydroxyapatite for 3h on 1000degC in air atmosphere The analysis of
CaSrNaP1000 spectrum (Fig 5c) confirms absence of O-H vibrations and presence of (P2O7)4-
vibrations which is unique for X2P2O7 types of compounds [43 44]
313 Morphology (SEM analysis) and element composition (EDS analysis)
SEM analysis confirms that all obtained samples using precipitation method have
crystallite size of nanometer dimensions In addition SEM images (Fig 6) show that the
particle size in all samples increases with the increase of temperature Namely nanoparticles
have a natural tendency to agglomerate for two reasons firstlyminusthe agglomeration is a more
sTab configuration from an energetic point of view and secondlyminusthe agglomeration allows
the crystallite growth [45 46]
Fig 6 SEM micrographs of A) CaP CaP400 and CaP1000 B) CaMgP CaMgP400 and
CaMgP1000 C) CaSrNaP CaSrNaP400 and CaSrNaP1000 with EDS spectra
Sintering at 1000 ordmC in air for 3 h led to a higher degree of densification of
CaSrNaP1000 (98 TD) than of CaMgP1000 (86 TD) The main reason for the differences in
density may be pore and particle size distribution ie different morphology (Fig 6) The
samples after calcinations at 400 ordmC during 3 h show more uniform particles and well ordered
structure Calcinations was done in order to allow better packing and lower inner activity and
it was expected that these samples may achieve higher density after sintering This was
achieved for CaSrNaP1000 sample where the grains are uniformly sintered and the contacts
between grains show hexagonal forms and the inserted section clearly demonstrates that we
have indeed obtained dense high temperature CaSrNaP material (Fig 6c) On the other hand
CaMgP1000 (Fig 6b) exhibits grain growth due to formation of agglomerated grains
Densification retards or inhibits wide pore distribution during densification the big pores
M Mirković et alScience of Sintering 50 (2018) 95-109 ___________________________________________________________________________
103
became larger while small pores shrink and disappear [47 48] SEM micrograph of
CaMgP1000 (Fig 6b) shows numerous agglomerates of hexagonal particles with plate like
morphology bonded together with average grain size between 2 and 5 microm with non-uniform
particles with many vacancies less compact particle packing higher inner activity To
confirm the chemical composition the EDS microanalysis was done on the CaP1000
CaMgP1000 and CaSrNaP1000 sintered samples (Fig 6) It revealed that chemical composition
very similar to nominal chemical composition According to semi quantitative chemical
analysis the mean values for the ratios CaPO for CaP1000 sample determined from four runs
are 21275 These ratios correspond to empirically calculated compound with nominal
composition-(Ca21P2O75) The mean value of the chemical ratios CaMgPO for CaMgP1000
was also determined from four runs and amounts to 120727 which corresponds to
empirical compound with following composition-(Ca12Mg07)19P2O7 The mean value of the
chemical ratios Ca Sr Na PO of the NaSrHA1000 sample was determined from four runs
and amounts to 0506227 which corresponds to-Ca05Sr06Na2P2O7 These results (Fig 6)
confirm the assumption that sintered samples based on hydroxyapatite at high temperature
(1000 ordmC) remain sTab and almost nominal chemical composition The obtained EDS results
are in good agreement with XRD results
32 Electrical conductivity
Recently ionic conductors with the apatite type structures attracted a great deal of
attention as an alternative electrolytes to the conventional yttria-stabilized zirconia to operate
in ITndashSOFCs in the intermediate temperature range (600-800 ordmC) [49-52] Namely
hydroxyapatite belongs to apatitic group of phosphate minerals with general formula
Me10(XO4)6Y2 with ionic substitution ability [53] Based on XRD (Tab I) unit cell
parameters of CaMgP samples are slightly increased compared to pure CaP sample the
contraction of a and c parameter can be attributed to much smaller ionic radius of Mg2+
than
Ca2+
[54] The change of unit cell parameters a and c of CaSrNaP indicate that Sr2+
and Na+
ions are incorporated in structure and not just adhered to the crystal where Sr2+
will firstly
replace Ca2+
ion in Ca2 site [55] On the other hand Na+ ion as a smaller than Ca
2+ could
possible replace Ca2+
in Ca1 site while a vacancy may formed at the OH- site [56] At 1000degC
structural transformation of hydroxyapatite occurs and it transforms to a high temperature
mixed phosphate ceramics with M2X2O7 structure where X ions are tetrahedral coordinated
(Fig 7)
Fig 7 Structures of M2X2O7 a) αndashSr2P2O7 b) βndashCa2P2O7 and c) Na4P2O7 (the green spheres
are Sr atoms blue spheres are Ca atoms the yellow spheres are Na atoms the red spheres are
O atoms and the purple spheres are P atoms)
The crystal structures of calcium strontium and sodium phosphate materials are
comprised of diphosphate ions which are linked to metallic ions to form a three
M Mirković et al Science of Sintering 50 (2018) 95-109
___________________________________________________________________________
104
dimensional network in β ndash Ca2P2O7 α-Sr2P2O7 and Na4P2O7 structures [57 58] A
common characteristic of thermal behavior of all phosphates is that under thermal
treatment they undergo dehydratation-condensation reaction which leads to phosphate
tetrahedral bonded to mutual oxygen atom The ion conductivity in these materials
occurs mainly via O- interstitials with preferential cndashaxis conduction [50 51 56 59]
in comparison to O2-
vacancy migration in fluorite and perovskitendashbased electrolytes
[59-63] Generally the impedance spectra are presented as negative of imaginary component
of impedance (ndashZimag) versus real component of impedance (Zreal) ie as Nyquist plot The
semicircles at high and intermediate frequency are ascribed to bulk and grain boundary
processes respectively while semicircle at low frequency represents the electrode process
contribution [56] In our case for the potential application in IT-SOFCs the measurements of
ionic conductivity of electrolytes in solid state of the CaMgP1000 and CaSrNaP1000 samples
were done in intermediate temperature range of 500-700 ordmC with the increments of 50 ordmC
The original Nyquist plots recorded in the available frequency range (1 Hz-100 kHz) are
presented in Fig 8 As it could be seen (Fig 8a and 8b) with increasing the temperature the
values of both resistance elements (Rb and Rgb) obviously decrease which causes an increase
in ωmax Consequently the whole region of the impedance points shifts towards the lowndash
frequency semicircle and at higher temperatures instead of Rb and Rgb separately only the
whole sum Rb+Rgb became readable in the available frequency range In this case at higher
temperatures the time constants associated with the bulk and grain boundary impedances are
much lower than those associated with the electrode interface As a result semicircles due to
bulk and grain boundary disappear and only a single semicircle due to electrode interfacial
processes can be observed [64 66] Therefore only the whole sum Rb+Rig became readable
and the values of total resistance were estimated from the experimental cross section of
obtained semicircles with the real component of impedance (Zreal) this intercept is marked by
arrows in Fig 8 (inset) New semicircles are observed to appear in a lowndashfrequency region
being particularly visible in temperature range at 650-700 ordmC (Fig 8b) Almost doubtless it
originates from the oxygen electrode reactions O2O2-
[64] which does not belong to the
scope of this study
20 30 40 50 60 700
10
20
30
40
50
60
70 a)
20 30 40 50 60 700
1
2
3
4
5
6
7
8
9
10x103
x103
-Zim
ag
(c
m)
Zreal (cm)
x102
x102
500 C
550 C
600 C
-Zim
ag
(c
m)
Zreal (cm)
6 8 10 12 14 160
2
4
6
8
10
12
14
16 b)x102
6 8 10 12 14 160
2
x102
x102
-Zim
ag
(c
m)
Zreal (cm)
x102
650 C
700 C
-Zim
ag
(c
m)
Zreal (cm)
Fig 8 (a)(b) Complex impedance plots of the sintered samples CaSrNaP1000 measured in the
temperature range from 500 to 700 ordmC in air atmosphere The arrows indicate the points on
the real axis corresponding to the readings Rb+Rgb
The total ionic conductivity of the sintered CaSrNaP1000 sample is shown in Tab II Obtained
values (at 700 ordmC amount to 418times10-3
Ω-1
cm-1
) are slightly higher than previously published
ones [64 66] More specifically values of ionic conductivity observed at 700 ordmC in this work
M Mirković et alScience of Sintering 50 (2018) 95-109 ___________________________________________________________________________
105
are similar to values obtained at 800 and 900 ordmC [67-71] This holds also if we compare the
results we found at lower temperatures with the literature data [64-67]
Tab II The temperature dependence of total ionic conductivity (κ) of the sintered CaMgP1000
and CaSrNaP1000 sample
COMPOSITION
κ
(Ω-1
cm-1
)
500 ordmC
κ
(Ω-1
cm-1
)
550 ordmC
κ
(Ω-1
cm-1
)
600 ordmC
κ
(Ω-1
cm-1
)
650 ordmC
κ
(Ω-1
cm-1
)
700 ordmC
CaMgP1000 123times10-4
222times10-4
358times10-4
294times10-4
312times10-4
CaSrNaP1000 227times10-4
356times10-4
657times10-4
117times10-3
190times10-3
According to the results listed in Tab II the dependence log κ = f(1T) of the sintered
sample CaSrNaP1000 is presented in Fig 9 The activation energy of conductivity (Ea) was
calculated from the Arrhenius plot according to the equation
Tk
EAT a 1
lnln (1)
where σ is the conductivity T is the absolute temperature A is the pre-exponential factor and
k is the Boltzmann constant Activation energy of total conductivity for sintered CaSrNaP1000
sample is 031 eV Ea values presented in our work are much lower than previously found [67
68 70 71] In addition comparing the obtained values of Ea with activation energies of
similar oxygen ion conductors containing dopants [71] obtained Ea values were significantly
lower We suggest that this is a consequence of wellndashordered structure and better process
ability of nanopowders obtained application method
100 105 110 115 120 125 130-38
-36
-34
-32
-30
-28
-26
Ea = 031 eV
log
k
1000T (K-1)
Fig 9 The dependence log κ = f(1T) of the sintered CaSrNaP1000 sample
M Mirković et al Science of Sintering 50 (2018) 95-109
___________________________________________________________________________
106
4 Conclusions
The pure Ca and doped with Mg and NaSr hydroxyapatite (CaP CaMgP and NaSrP)
were successfully synthesized using slightly modified solution precipitation method (partial
substitution nitrates by acetate solutions) and thus we achieved higher solubility and more
cost effective synthesis Any secondary phase that formed during synthesis decomposed
during thermal treatment leaving pure CaP CaMgP and CaSrNaP hexagonal (P63m)
Ca5(PO4)3OH materials at room temperature During sintering process at 1000 ordmC in an air for
3 h we developed high temperature doped calcium phosphate materials ie CaMgP1000 and
CaSrNaP1000 materials SEM analysis showed that CaMgP1000 and CaSrNaP1000 materials have
high structural arrangement EDS analysis confirmed that we obtained phosphate compound
type-M2X2O7 The fine crystalline powders after sintering at 1000 ordmC achieved 98 of
theoretical density The maximum value of electrical conductivity at 700 ordmC was achieved for
CaSrNaP1000 material with amounts to 190times10-3
Ω-1
cm-1
Electrical characteristics of the
sintered samples indicate the superiority of solid solution for the application in SOFCs
Additionally considering high sintering temperature one can conclude that the obtained
phosphate material demonstrates high thermal stability an important characteristics in its
potential use as solid electrolyte However it is important to note that the future research
should include furthering lower temperature synthesis routes improving stability ITndashSOFCs
electrolyteelectrode molecular dynamics (MD) simulations of conduction mechanisms and
the general search for improved ion conductors Also the breadth of possible doping regimes
in this phosphate material provides new opportunities to design and optimize their transport
parameters for potential clean energy applications
Acknowledgements
This work was supported by the Ministry of Education Science and Technological
development of Serbia as part of Projects III 45012 and OI 176016
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Садржај Метода таложне титрације коришћена је за добијање чистог и допираног
хидроксиапатита јонима Sr Mg и Na (CaP CaMgP and CaSrNaP) За синтезу
супституисаних хидроксиапатитских материјала коришћена је модификована
таложна метода у односу на досада познате методе Због боље растворљивости и
знатно веће исплативости коришћени су раствори ацетата уместо нитрата
Синтетисани материјали су калцинисани на 400 degC а затим окарактерисани
рендгенском дифракцијом инфра црвеном спектроскопијом са Фуријеовом
трансформацијом и скенирајућом електронском микроскопијом Калцинисани узорци
су синтеровани на 1000 degC са задржавањем од 3 сата у атмосфери ваздуха (CaP1000
CaMgP1000 and CaSrNaP1000) Синтеровани материјали окарактерисани су рендгенском
дифракцијом на поликристалном узорку инфрацрвеном спекторскопијом са
Фуријеовом трансформацијом и скенирајућом електронском микроскопијом са
M Mirković et alScience of Sintering 50 (2018) 95-109 ___________________________________________________________________________
109
енергетско дисперзивном анализом као и методе комплексне импендансе Највећа
проводљивост добијена је на температури од 700 degC и износи 190times10-3
Ω-1
cm-1
за
мешовито допирани фосфатни материјал (CaSrNaP1000) Добијене активационе
енергије проводљивости износе 031 eV у температурном опсегу од 500-700 ordmC
Кључне речи Синтеровање Рендгенска дифракција Електрична проводљивост
Фосфатни материјали Горивне ћелије
copy 2016 Authors Published by the International Institute for the Science of Sintering This
article is an open access article distributed under the terms and conditions of the Creative
Commons mdash Attribution 40 International license
(httpscreativecommonsorglicensesby40)
M Mirković et alScience of Sintering 50 (2018) 95-109 ___________________________________________________________________________
101
Furthermore the FTIR spectrum of CaMgP shows a slight indication of 1410 cm-1
carbonate band (Fig 4a) thus indicating no significant presence of CO3 group in CaMgP
powder [40] Weak band of MgHCO3 can be noticed around 1634 cm-1
[36] CaSrNaP band at
1385 cm-1
indicates incorporation of Sr2+
and Na+ ions into the structure of hydroxylapatite
(Fig 4c) [35 38] A band that indicates carbonate group (coming from the reaction medium)
in the structure of CaSrNaP is demonstrated by bands at 1417 and 1639 cm-1
[41] FTIR
spectra confirm previously described XRD results that using acetate solution multi doped
hydroxyapatite material can be obtained
FTIR spectra of thermally treated samples at 1000 ordmC for 3 h in air are shown in Fig
5 also shows presence of the characteristic signals of apatite [36] CaMgP1000 (Fig 5a)
sample shows bands in spectral regions 1470-870 cm-1
and 605-560 cm-1
which are generally
related to P-O species with different phosphorous configuration [42 43] Characteristic
vibrations of Mg2P2O7 phase are bands at 1050 605 and 560 cm-1
[42] The bands at around
1050 cm-1
belong to triply degenerate asymmetric P-O stretching mode while the band
around 955 cm-1
belongs to P-O stretching mode these bands belong to β-Ca2P2O7 phase [43]
There is no evidence of any characteristic hydroxyl group vibrations (631 and 3572 cm-1
)
CaP1000 (Fig 5b) bands are weaker than in untreated samples indicating lack of water
vibrations in structure The bands around 600 and 580 cm-1
are attributed to different types of
P-O-P bending mode vibrations which belong to β-Ca2P2O7 [43] Figure 5c shows vibration
spectrum of CaSrNaP1000 material Presence of previously described P-O and P-O-P vibrations
which belong to β-Ca2P2O7 phase also belong to Sr2P2O7 [43 44] The band around 740 cm-1
belongs to P-O-P bridge stretching vibration a striking feature of (P2O7)4-
ions in Sr2P2O7
FTIR spectrum of CaSrNaP (Fig 5c) shows no evidence of O-H stretching modes at around
3500 and 1600 cm-1
[44] Metal-oxygen vibrations in these types of systems are generally too
low to be seen in the 400-4000 cm-1
region
Fig 5 FTIR specta of thermally treated samples at 1000 degC in air for 3 h a) CaMgP1000 b)
CaP1000 and c) CaSrNaP1000
M Mirković et al Science of Sintering 50 (2018) 95-109
___________________________________________________________________________
102
Based on FTIR analysis at 1000 degC high temperature CaMgP1000 material is obtained
All of identified vibrations that belong to CaP1000 material (Fig 5b) confirm that β-Ca2P2O7 is
obtained by sintering hydroxyapatite for 3h on 1000degC in air atmosphere The analysis of
CaSrNaP1000 spectrum (Fig 5c) confirms absence of O-H vibrations and presence of (P2O7)4-
vibrations which is unique for X2P2O7 types of compounds [43 44]
313 Morphology (SEM analysis) and element composition (EDS analysis)
SEM analysis confirms that all obtained samples using precipitation method have
crystallite size of nanometer dimensions In addition SEM images (Fig 6) show that the
particle size in all samples increases with the increase of temperature Namely nanoparticles
have a natural tendency to agglomerate for two reasons firstlyminusthe agglomeration is a more
sTab configuration from an energetic point of view and secondlyminusthe agglomeration allows
the crystallite growth [45 46]
Fig 6 SEM micrographs of A) CaP CaP400 and CaP1000 B) CaMgP CaMgP400 and
CaMgP1000 C) CaSrNaP CaSrNaP400 and CaSrNaP1000 with EDS spectra
Sintering at 1000 ordmC in air for 3 h led to a higher degree of densification of
CaSrNaP1000 (98 TD) than of CaMgP1000 (86 TD) The main reason for the differences in
density may be pore and particle size distribution ie different morphology (Fig 6) The
samples after calcinations at 400 ordmC during 3 h show more uniform particles and well ordered
structure Calcinations was done in order to allow better packing and lower inner activity and
it was expected that these samples may achieve higher density after sintering This was
achieved for CaSrNaP1000 sample where the grains are uniformly sintered and the contacts
between grains show hexagonal forms and the inserted section clearly demonstrates that we
have indeed obtained dense high temperature CaSrNaP material (Fig 6c) On the other hand
CaMgP1000 (Fig 6b) exhibits grain growth due to formation of agglomerated grains
Densification retards or inhibits wide pore distribution during densification the big pores
M Mirković et alScience of Sintering 50 (2018) 95-109 ___________________________________________________________________________
103
became larger while small pores shrink and disappear [47 48] SEM micrograph of
CaMgP1000 (Fig 6b) shows numerous agglomerates of hexagonal particles with plate like
morphology bonded together with average grain size between 2 and 5 microm with non-uniform
particles with many vacancies less compact particle packing higher inner activity To
confirm the chemical composition the EDS microanalysis was done on the CaP1000
CaMgP1000 and CaSrNaP1000 sintered samples (Fig 6) It revealed that chemical composition
very similar to nominal chemical composition According to semi quantitative chemical
analysis the mean values for the ratios CaPO for CaP1000 sample determined from four runs
are 21275 These ratios correspond to empirically calculated compound with nominal
composition-(Ca21P2O75) The mean value of the chemical ratios CaMgPO for CaMgP1000
was also determined from four runs and amounts to 120727 which corresponds to
empirical compound with following composition-(Ca12Mg07)19P2O7 The mean value of the
chemical ratios Ca Sr Na PO of the NaSrHA1000 sample was determined from four runs
and amounts to 0506227 which corresponds to-Ca05Sr06Na2P2O7 These results (Fig 6)
confirm the assumption that sintered samples based on hydroxyapatite at high temperature
(1000 ordmC) remain sTab and almost nominal chemical composition The obtained EDS results
are in good agreement with XRD results
32 Electrical conductivity
Recently ionic conductors with the apatite type structures attracted a great deal of
attention as an alternative electrolytes to the conventional yttria-stabilized zirconia to operate
in ITndashSOFCs in the intermediate temperature range (600-800 ordmC) [49-52] Namely
hydroxyapatite belongs to apatitic group of phosphate minerals with general formula
Me10(XO4)6Y2 with ionic substitution ability [53] Based on XRD (Tab I) unit cell
parameters of CaMgP samples are slightly increased compared to pure CaP sample the
contraction of a and c parameter can be attributed to much smaller ionic radius of Mg2+
than
Ca2+
[54] The change of unit cell parameters a and c of CaSrNaP indicate that Sr2+
and Na+
ions are incorporated in structure and not just adhered to the crystal where Sr2+
will firstly
replace Ca2+
ion in Ca2 site [55] On the other hand Na+ ion as a smaller than Ca
2+ could
possible replace Ca2+
in Ca1 site while a vacancy may formed at the OH- site [56] At 1000degC
structural transformation of hydroxyapatite occurs and it transforms to a high temperature
mixed phosphate ceramics with M2X2O7 structure where X ions are tetrahedral coordinated
(Fig 7)
Fig 7 Structures of M2X2O7 a) αndashSr2P2O7 b) βndashCa2P2O7 and c) Na4P2O7 (the green spheres
are Sr atoms blue spheres are Ca atoms the yellow spheres are Na atoms the red spheres are
O atoms and the purple spheres are P atoms)
The crystal structures of calcium strontium and sodium phosphate materials are
comprised of diphosphate ions which are linked to metallic ions to form a three
M Mirković et al Science of Sintering 50 (2018) 95-109
___________________________________________________________________________
104
dimensional network in β ndash Ca2P2O7 α-Sr2P2O7 and Na4P2O7 structures [57 58] A
common characteristic of thermal behavior of all phosphates is that under thermal
treatment they undergo dehydratation-condensation reaction which leads to phosphate
tetrahedral bonded to mutual oxygen atom The ion conductivity in these materials
occurs mainly via O- interstitials with preferential cndashaxis conduction [50 51 56 59]
in comparison to O2-
vacancy migration in fluorite and perovskitendashbased electrolytes
[59-63] Generally the impedance spectra are presented as negative of imaginary component
of impedance (ndashZimag) versus real component of impedance (Zreal) ie as Nyquist plot The
semicircles at high and intermediate frequency are ascribed to bulk and grain boundary
processes respectively while semicircle at low frequency represents the electrode process
contribution [56] In our case for the potential application in IT-SOFCs the measurements of
ionic conductivity of electrolytes in solid state of the CaMgP1000 and CaSrNaP1000 samples
were done in intermediate temperature range of 500-700 ordmC with the increments of 50 ordmC
The original Nyquist plots recorded in the available frequency range (1 Hz-100 kHz) are
presented in Fig 8 As it could be seen (Fig 8a and 8b) with increasing the temperature the
values of both resistance elements (Rb and Rgb) obviously decrease which causes an increase
in ωmax Consequently the whole region of the impedance points shifts towards the lowndash
frequency semicircle and at higher temperatures instead of Rb and Rgb separately only the
whole sum Rb+Rgb became readable in the available frequency range In this case at higher
temperatures the time constants associated with the bulk and grain boundary impedances are
much lower than those associated with the electrode interface As a result semicircles due to
bulk and grain boundary disappear and only a single semicircle due to electrode interfacial
processes can be observed [64 66] Therefore only the whole sum Rb+Rig became readable
and the values of total resistance were estimated from the experimental cross section of
obtained semicircles with the real component of impedance (Zreal) this intercept is marked by
arrows in Fig 8 (inset) New semicircles are observed to appear in a lowndashfrequency region
being particularly visible in temperature range at 650-700 ordmC (Fig 8b) Almost doubtless it
originates from the oxygen electrode reactions O2O2-
[64] which does not belong to the
scope of this study
20 30 40 50 60 700
10
20
30
40
50
60
70 a)
20 30 40 50 60 700
1
2
3
4
5
6
7
8
9
10x103
x103
-Zim
ag
(c
m)
Zreal (cm)
x102
x102
500 C
550 C
600 C
-Zim
ag
(c
m)
Zreal (cm)
6 8 10 12 14 160
2
4
6
8
10
12
14
16 b)x102
6 8 10 12 14 160
2
x102
x102
-Zim
ag
(c
m)
Zreal (cm)
x102
650 C
700 C
-Zim
ag
(c
m)
Zreal (cm)
Fig 8 (a)(b) Complex impedance plots of the sintered samples CaSrNaP1000 measured in the
temperature range from 500 to 700 ordmC in air atmosphere The arrows indicate the points on
the real axis corresponding to the readings Rb+Rgb
The total ionic conductivity of the sintered CaSrNaP1000 sample is shown in Tab II Obtained
values (at 700 ordmC amount to 418times10-3
Ω-1
cm-1
) are slightly higher than previously published
ones [64 66] More specifically values of ionic conductivity observed at 700 ordmC in this work
M Mirković et alScience of Sintering 50 (2018) 95-109 ___________________________________________________________________________
105
are similar to values obtained at 800 and 900 ordmC [67-71] This holds also if we compare the
results we found at lower temperatures with the literature data [64-67]
Tab II The temperature dependence of total ionic conductivity (κ) of the sintered CaMgP1000
and CaSrNaP1000 sample
COMPOSITION
κ
(Ω-1
cm-1
)
500 ordmC
κ
(Ω-1
cm-1
)
550 ordmC
κ
(Ω-1
cm-1
)
600 ordmC
κ
(Ω-1
cm-1
)
650 ordmC
κ
(Ω-1
cm-1
)
700 ordmC
CaMgP1000 123times10-4
222times10-4
358times10-4
294times10-4
312times10-4
CaSrNaP1000 227times10-4
356times10-4
657times10-4
117times10-3
190times10-3
According to the results listed in Tab II the dependence log κ = f(1T) of the sintered
sample CaSrNaP1000 is presented in Fig 9 The activation energy of conductivity (Ea) was
calculated from the Arrhenius plot according to the equation
Tk
EAT a 1
lnln (1)
where σ is the conductivity T is the absolute temperature A is the pre-exponential factor and
k is the Boltzmann constant Activation energy of total conductivity for sintered CaSrNaP1000
sample is 031 eV Ea values presented in our work are much lower than previously found [67
68 70 71] In addition comparing the obtained values of Ea with activation energies of
similar oxygen ion conductors containing dopants [71] obtained Ea values were significantly
lower We suggest that this is a consequence of wellndashordered structure and better process
ability of nanopowders obtained application method
100 105 110 115 120 125 130-38
-36
-34
-32
-30
-28
-26
Ea = 031 eV
log
k
1000T (K-1)
Fig 9 The dependence log κ = f(1T) of the sintered CaSrNaP1000 sample
M Mirković et al Science of Sintering 50 (2018) 95-109
___________________________________________________________________________
106
4 Conclusions
The pure Ca and doped with Mg and NaSr hydroxyapatite (CaP CaMgP and NaSrP)
were successfully synthesized using slightly modified solution precipitation method (partial
substitution nitrates by acetate solutions) and thus we achieved higher solubility and more
cost effective synthesis Any secondary phase that formed during synthesis decomposed
during thermal treatment leaving pure CaP CaMgP and CaSrNaP hexagonal (P63m)
Ca5(PO4)3OH materials at room temperature During sintering process at 1000 ordmC in an air for
3 h we developed high temperature doped calcium phosphate materials ie CaMgP1000 and
CaSrNaP1000 materials SEM analysis showed that CaMgP1000 and CaSrNaP1000 materials have
high structural arrangement EDS analysis confirmed that we obtained phosphate compound
type-M2X2O7 The fine crystalline powders after sintering at 1000 ordmC achieved 98 of
theoretical density The maximum value of electrical conductivity at 700 ordmC was achieved for
CaSrNaP1000 material with amounts to 190times10-3
Ω-1
cm-1
Electrical characteristics of the
sintered samples indicate the superiority of solid solution for the application in SOFCs
Additionally considering high sintering temperature one can conclude that the obtained
phosphate material demonstrates high thermal stability an important characteristics in its
potential use as solid electrolyte However it is important to note that the future research
should include furthering lower temperature synthesis routes improving stability ITndashSOFCs
electrolyteelectrode molecular dynamics (MD) simulations of conduction mechanisms and
the general search for improved ion conductors Also the breadth of possible doping regimes
in this phosphate material provides new opportunities to design and optimize their transport
parameters for potential clean energy applications
Acknowledgements
This work was supported by the Ministry of Education Science and Technological
development of Serbia as part of Projects III 45012 and OI 176016
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Nuacutentildeez M A G Aranda J Power Sources 195 (2010) 2496
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Losilla Journal of Power Sources 245 (2014) 107
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Садржај Метода таложне титрације коришћена је за добијање чистог и допираног
хидроксиапатита јонима Sr Mg и Na (CaP CaMgP and CaSrNaP) За синтезу
супституисаних хидроксиапатитских материјала коришћена је модификована
таложна метода у односу на досада познате методе Због боље растворљивости и
знатно веће исплативости коришћени су раствори ацетата уместо нитрата
Синтетисани материјали су калцинисани на 400 degC а затим окарактерисани
рендгенском дифракцијом инфра црвеном спектроскопијом са Фуријеовом
трансформацијом и скенирајућом електронском микроскопијом Калцинисани узорци
су синтеровани на 1000 degC са задржавањем од 3 сата у атмосфери ваздуха (CaP1000
CaMgP1000 and CaSrNaP1000) Синтеровани материјали окарактерисани су рендгенском
дифракцијом на поликристалном узорку инфрацрвеном спекторскопијом са
Фуријеовом трансформацијом и скенирајућом електронском микроскопијом са
M Mirković et alScience of Sintering 50 (2018) 95-109 ___________________________________________________________________________
109
енергетско дисперзивном анализом као и методе комплексне импендансе Највећа
проводљивост добијена је на температури од 700 degC и износи 190times10-3
Ω-1
cm-1
за
мешовито допирани фосфатни материјал (CaSrNaP1000) Добијене активационе
енергије проводљивости износе 031 eV у температурном опсегу од 500-700 ordmC
Кључне речи Синтеровање Рендгенска дифракција Електрична проводљивост
Фосфатни материјали Горивне ћелије
copy 2016 Authors Published by the International Institute for the Science of Sintering This
article is an open access article distributed under the terms and conditions of the Creative
Commons mdash Attribution 40 International license
(httpscreativecommonsorglicensesby40)
M Mirković et al Science of Sintering 50 (2018) 95-109
___________________________________________________________________________
102
Based on FTIR analysis at 1000 degC high temperature CaMgP1000 material is obtained
All of identified vibrations that belong to CaP1000 material (Fig 5b) confirm that β-Ca2P2O7 is
obtained by sintering hydroxyapatite for 3h on 1000degC in air atmosphere The analysis of
CaSrNaP1000 spectrum (Fig 5c) confirms absence of O-H vibrations and presence of (P2O7)4-
vibrations which is unique for X2P2O7 types of compounds [43 44]
313 Morphology (SEM analysis) and element composition (EDS analysis)
SEM analysis confirms that all obtained samples using precipitation method have
crystallite size of nanometer dimensions In addition SEM images (Fig 6) show that the
particle size in all samples increases with the increase of temperature Namely nanoparticles
have a natural tendency to agglomerate for two reasons firstlyminusthe agglomeration is a more
sTab configuration from an energetic point of view and secondlyminusthe agglomeration allows
the crystallite growth [45 46]
Fig 6 SEM micrographs of A) CaP CaP400 and CaP1000 B) CaMgP CaMgP400 and
CaMgP1000 C) CaSrNaP CaSrNaP400 and CaSrNaP1000 with EDS spectra
Sintering at 1000 ordmC in air for 3 h led to a higher degree of densification of
CaSrNaP1000 (98 TD) than of CaMgP1000 (86 TD) The main reason for the differences in
density may be pore and particle size distribution ie different morphology (Fig 6) The
samples after calcinations at 400 ordmC during 3 h show more uniform particles and well ordered
structure Calcinations was done in order to allow better packing and lower inner activity and
it was expected that these samples may achieve higher density after sintering This was
achieved for CaSrNaP1000 sample where the grains are uniformly sintered and the contacts
between grains show hexagonal forms and the inserted section clearly demonstrates that we
have indeed obtained dense high temperature CaSrNaP material (Fig 6c) On the other hand
CaMgP1000 (Fig 6b) exhibits grain growth due to formation of agglomerated grains
Densification retards or inhibits wide pore distribution during densification the big pores
M Mirković et alScience of Sintering 50 (2018) 95-109 ___________________________________________________________________________
103
became larger while small pores shrink and disappear [47 48] SEM micrograph of
CaMgP1000 (Fig 6b) shows numerous agglomerates of hexagonal particles with plate like
morphology bonded together with average grain size between 2 and 5 microm with non-uniform
particles with many vacancies less compact particle packing higher inner activity To
confirm the chemical composition the EDS microanalysis was done on the CaP1000
CaMgP1000 and CaSrNaP1000 sintered samples (Fig 6) It revealed that chemical composition
very similar to nominal chemical composition According to semi quantitative chemical
analysis the mean values for the ratios CaPO for CaP1000 sample determined from four runs
are 21275 These ratios correspond to empirically calculated compound with nominal
composition-(Ca21P2O75) The mean value of the chemical ratios CaMgPO for CaMgP1000
was also determined from four runs and amounts to 120727 which corresponds to
empirical compound with following composition-(Ca12Mg07)19P2O7 The mean value of the
chemical ratios Ca Sr Na PO of the NaSrHA1000 sample was determined from four runs
and amounts to 0506227 which corresponds to-Ca05Sr06Na2P2O7 These results (Fig 6)
confirm the assumption that sintered samples based on hydroxyapatite at high temperature
(1000 ordmC) remain sTab and almost nominal chemical composition The obtained EDS results
are in good agreement with XRD results
32 Electrical conductivity
Recently ionic conductors with the apatite type structures attracted a great deal of
attention as an alternative electrolytes to the conventional yttria-stabilized zirconia to operate
in ITndashSOFCs in the intermediate temperature range (600-800 ordmC) [49-52] Namely
hydroxyapatite belongs to apatitic group of phosphate minerals with general formula
Me10(XO4)6Y2 with ionic substitution ability [53] Based on XRD (Tab I) unit cell
parameters of CaMgP samples are slightly increased compared to pure CaP sample the
contraction of a and c parameter can be attributed to much smaller ionic radius of Mg2+
than
Ca2+
[54] The change of unit cell parameters a and c of CaSrNaP indicate that Sr2+
and Na+
ions are incorporated in structure and not just adhered to the crystal where Sr2+
will firstly
replace Ca2+
ion in Ca2 site [55] On the other hand Na+ ion as a smaller than Ca
2+ could
possible replace Ca2+
in Ca1 site while a vacancy may formed at the OH- site [56] At 1000degC
structural transformation of hydroxyapatite occurs and it transforms to a high temperature
mixed phosphate ceramics with M2X2O7 structure where X ions are tetrahedral coordinated
(Fig 7)
Fig 7 Structures of M2X2O7 a) αndashSr2P2O7 b) βndashCa2P2O7 and c) Na4P2O7 (the green spheres
are Sr atoms blue spheres are Ca atoms the yellow spheres are Na atoms the red spheres are
O atoms and the purple spheres are P atoms)
The crystal structures of calcium strontium and sodium phosphate materials are
comprised of diphosphate ions which are linked to metallic ions to form a three
M Mirković et al Science of Sintering 50 (2018) 95-109
___________________________________________________________________________
104
dimensional network in β ndash Ca2P2O7 α-Sr2P2O7 and Na4P2O7 structures [57 58] A
common characteristic of thermal behavior of all phosphates is that under thermal
treatment they undergo dehydratation-condensation reaction which leads to phosphate
tetrahedral bonded to mutual oxygen atom The ion conductivity in these materials
occurs mainly via O- interstitials with preferential cndashaxis conduction [50 51 56 59]
in comparison to O2-
vacancy migration in fluorite and perovskitendashbased electrolytes
[59-63] Generally the impedance spectra are presented as negative of imaginary component
of impedance (ndashZimag) versus real component of impedance (Zreal) ie as Nyquist plot The
semicircles at high and intermediate frequency are ascribed to bulk and grain boundary
processes respectively while semicircle at low frequency represents the electrode process
contribution [56] In our case for the potential application in IT-SOFCs the measurements of
ionic conductivity of electrolytes in solid state of the CaMgP1000 and CaSrNaP1000 samples
were done in intermediate temperature range of 500-700 ordmC with the increments of 50 ordmC
The original Nyquist plots recorded in the available frequency range (1 Hz-100 kHz) are
presented in Fig 8 As it could be seen (Fig 8a and 8b) with increasing the temperature the
values of both resistance elements (Rb and Rgb) obviously decrease which causes an increase
in ωmax Consequently the whole region of the impedance points shifts towards the lowndash
frequency semicircle and at higher temperatures instead of Rb and Rgb separately only the
whole sum Rb+Rgb became readable in the available frequency range In this case at higher
temperatures the time constants associated with the bulk and grain boundary impedances are
much lower than those associated with the electrode interface As a result semicircles due to
bulk and grain boundary disappear and only a single semicircle due to electrode interfacial
processes can be observed [64 66] Therefore only the whole sum Rb+Rig became readable
and the values of total resistance were estimated from the experimental cross section of
obtained semicircles with the real component of impedance (Zreal) this intercept is marked by
arrows in Fig 8 (inset) New semicircles are observed to appear in a lowndashfrequency region
being particularly visible in temperature range at 650-700 ordmC (Fig 8b) Almost doubtless it
originates from the oxygen electrode reactions O2O2-
[64] which does not belong to the
scope of this study
20 30 40 50 60 700
10
20
30
40
50
60
70 a)
20 30 40 50 60 700
1
2
3
4
5
6
7
8
9
10x103
x103
-Zim
ag
(c
m)
Zreal (cm)
x102
x102
500 C
550 C
600 C
-Zim
ag
(c
m)
Zreal (cm)
6 8 10 12 14 160
2
4
6
8
10
12
14
16 b)x102
6 8 10 12 14 160
2
x102
x102
-Zim
ag
(c
m)
Zreal (cm)
x102
650 C
700 C
-Zim
ag
(c
m)
Zreal (cm)
Fig 8 (a)(b) Complex impedance plots of the sintered samples CaSrNaP1000 measured in the
temperature range from 500 to 700 ordmC in air atmosphere The arrows indicate the points on
the real axis corresponding to the readings Rb+Rgb
The total ionic conductivity of the sintered CaSrNaP1000 sample is shown in Tab II Obtained
values (at 700 ordmC amount to 418times10-3
Ω-1
cm-1
) are slightly higher than previously published
ones [64 66] More specifically values of ionic conductivity observed at 700 ordmC in this work
M Mirković et alScience of Sintering 50 (2018) 95-109 ___________________________________________________________________________
105
are similar to values obtained at 800 and 900 ordmC [67-71] This holds also if we compare the
results we found at lower temperatures with the literature data [64-67]
Tab II The temperature dependence of total ionic conductivity (κ) of the sintered CaMgP1000
and CaSrNaP1000 sample
COMPOSITION
κ
(Ω-1
cm-1
)
500 ordmC
κ
(Ω-1
cm-1
)
550 ordmC
κ
(Ω-1
cm-1
)
600 ordmC
κ
(Ω-1
cm-1
)
650 ordmC
κ
(Ω-1
cm-1
)
700 ordmC
CaMgP1000 123times10-4
222times10-4
358times10-4
294times10-4
312times10-4
CaSrNaP1000 227times10-4
356times10-4
657times10-4
117times10-3
190times10-3
According to the results listed in Tab II the dependence log κ = f(1T) of the sintered
sample CaSrNaP1000 is presented in Fig 9 The activation energy of conductivity (Ea) was
calculated from the Arrhenius plot according to the equation
Tk
EAT a 1
lnln (1)
where σ is the conductivity T is the absolute temperature A is the pre-exponential factor and
k is the Boltzmann constant Activation energy of total conductivity for sintered CaSrNaP1000
sample is 031 eV Ea values presented in our work are much lower than previously found [67
68 70 71] In addition comparing the obtained values of Ea with activation energies of
similar oxygen ion conductors containing dopants [71] obtained Ea values were significantly
lower We suggest that this is a consequence of wellndashordered structure and better process
ability of nanopowders obtained application method
100 105 110 115 120 125 130-38
-36
-34
-32
-30
-28
-26
Ea = 031 eV
log
k
1000T (K-1)
Fig 9 The dependence log κ = f(1T) of the sintered CaSrNaP1000 sample
M Mirković et al Science of Sintering 50 (2018) 95-109
___________________________________________________________________________
106
4 Conclusions
The pure Ca and doped with Mg and NaSr hydroxyapatite (CaP CaMgP and NaSrP)
were successfully synthesized using slightly modified solution precipitation method (partial
substitution nitrates by acetate solutions) and thus we achieved higher solubility and more
cost effective synthesis Any secondary phase that formed during synthesis decomposed
during thermal treatment leaving pure CaP CaMgP and CaSrNaP hexagonal (P63m)
Ca5(PO4)3OH materials at room temperature During sintering process at 1000 ordmC in an air for
3 h we developed high temperature doped calcium phosphate materials ie CaMgP1000 and
CaSrNaP1000 materials SEM analysis showed that CaMgP1000 and CaSrNaP1000 materials have
high structural arrangement EDS analysis confirmed that we obtained phosphate compound
type-M2X2O7 The fine crystalline powders after sintering at 1000 ordmC achieved 98 of
theoretical density The maximum value of electrical conductivity at 700 ordmC was achieved for
CaSrNaP1000 material with amounts to 190times10-3
Ω-1
cm-1
Electrical characteristics of the
sintered samples indicate the superiority of solid solution for the application in SOFCs
Additionally considering high sintering temperature one can conclude that the obtained
phosphate material demonstrates high thermal stability an important characteristics in its
potential use as solid electrolyte However it is important to note that the future research
should include furthering lower temperature synthesis routes improving stability ITndashSOFCs
electrolyteelectrode molecular dynamics (MD) simulations of conduction mechanisms and
the general search for improved ion conductors Also the breadth of possible doping regimes
in this phosphate material provides new opportunities to design and optimize their transport
parameters for potential clean energy applications
Acknowledgements
This work was supported by the Ministry of Education Science and Technological
development of Serbia as part of Projects III 45012 and OI 176016
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Bogdanović Mat Chem Phys 153 (2015) 422
10 M Stojmenović M Žunić J Gulicovski D Bajuk-Bogdanović I Holclajtner-
Antunović V Dodevski J Mater Sci 50 (2015) 3781
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107
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28 (1988) 660
13 F Bakan O Laccedilin H Sarac Powder Technol 233 (2013) 295
14 M Sadat-Shojai M-T Khorasani A Jamshidi J Cryst Growth 361 (2012) 73
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16 I Mobasherpour MS Heshajin A Kazemzadeh M Zakeri J Alloys Comp 430
(2007) 330
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18 T T Trang Pham T Phuong Nguyen T Nam Pham T Phuong Vu D L Tran
HThai Vietnam Academy of Science and Technology 4 (2013) 1
19 S C Cox P Jamshidi L M Grover K K Mallick Mat Sci Eng C-Bio S 35
(2014) 106
20 E Champion Acta Biomater 9 (2013) 5855
21 S J Kalita A Bhardwaj H A Bhatt Mat Sci Eng C-Bio S 27 (2007) 441
22 S Ramesh K L Aw R Tolouei M Amiriyan C Y Tan M Hamdi Ceram Int
39 (2013) 111
23 Đ Veljović G Vuković I Steins E Palcevskis P S Usković R Petrović Sci
Sinter 45 (2013) 233
24 S Raynaud E Champion D Bernache-Assollant Biomater 23 (2002) 1073
25 Rigaku Corporation Tokyo J PDXL Version 2030 Integrated X-ray Powder
Diffraction Software 2011
26 Powder Diffraction File P-D announcement of new data-base release 2012
International Centrefor Diffraction Data (ICDD)
27 K Momma F Izumi J Appl Crystallogr 44 (2011) 1272
28 M Prekajski M Mirković B Todorović A Matković M Marinović-Cincović J
Luković B Matović J Eur CeramSoc 36 (2016) 1293
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32 G Qi S Zhang K A Khor C Liu X Zeng W Weng Thin Solid Films 516
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33 S C Cox P Jamshidi L M Grover K K Mallick Mat Sci Eng C-Bio S 35
(2014) 106
34 A Cuccu S Montinaro R Orrugrave G Cao D Bellucci A Sola Ceram Int 41
(2015)725
35 W Zhang Y Shen H Pan K Lin X Liu B W Darvell Acta Biomater 7 (2011)
800
36 J A Gadsden Infrared spectra of minerals and related inorganic compounds
Butterworths London 1975
37 M Iafisco A Ruffini A Adamiano S Sprio A Tampieri Mat Sci Eng C-Bio S
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38 S Koutsopoulos J Biomed Mater Res 62 (2002) 600
39 S-C Liou S-Y Chen H-Y Lee J-S Bow Biomaterials 25 (2004) 189
40 M E Fleet Biomaterials 30 (2009) 1473
41 N V Chukanov Infrared spectra of mineral species Extended library Springer
London Limited 2013
42 V Koleva V Stefov M Najdoski A Cahil Thermochim Acta 619 (2015) 20
43 B Mehdikhani G H Borhani J Ceram Process Res 16 (2015) 308
44 P Solanki S Vasant M Joshi Int J Appl Ceram Tech 11 (2014) 663
45 G B Balazs R Glass Solid State Ionics 76 (1995) 155
46 G Buvaneswari K Valsalan Mater Lett 134 (2014) 252
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47 A Farzadi F Bakhshi M Solati-Hashjin M Asadi-Eydivand N A Osman Ceram
Int40 (2014) 6021
48 A G Evans C H Hsueh J Am Ceram Soc 69 (1986) 444
49 A J Jacobson Chem Mater 22 (2010) 660
50 A Orera P R Slater Chem Mater 22 (2010) 675
51 L Malavasi C A J Fisher M S Islam Chem Soc Review 39 (2010) 4370
52 D Marrero-Loacutepez M C Martiacuten-Sedentildeo J Pentildea-Martiacutenez J C Ruiz-Morales P
Nuacutentildeez M A G Aranda J Power Sources 195 (2010) 2496
53 M P Moreira G D de Almeida Soares J Dentzer K Anselme L Aacute de Sena A
Kuznetsov Mat Sci Eng C-Bio S 61 (2016) 736
54 F Ren Y Leng R Xin X Ge Acta Biomater 6 (2010) 2787
55 Z Y Li W M Lam C Yang B Xu G X Ni S A Abbah Biomaterials 28
(2007)1452
56 Z Leilei L Hejun L Kezhi F Qiangang Z Yulei L Shoujie Ceram Int 40
(2014) 13123
57 K Y Leung C Calvo Can J Chem 50 (1972) 2519
58 H Larsove J Ingela M Christina Acta Chemica Scandinavica 22 (1968) 1419
59 E Kendrick M S Islam P R Slater J Mater Chem 17 (2007) 3104
60 E Kendrick M S Islam P R Slater Chem Commun 715 (2008) 7
61 K Fukuda T Asaka R Hamaguchi T Suzuki H Oka A Berghout Chem Mater
23 (2011) 5474
62 E Kendrick M S Islam P R Slater Solid State Ionics 177 (2007) 3411
63 L Leon-Reina E R Losilla M Martinez-Lara S Bruque M A G J Mater
Chem 14 (2004) 1142
64 N Nallamuthu I Prakash N Satyanarayana M Venkateswarlu J Alloys and
Compounds 509 (2011) 1138
65 M Sato Y Kono H Ueda K Uematsu K Toda Solid State Ionics 83 (1996)249
66 D Marrero-Loacutepez L dos Santos-Goacutemez L Leoacuten-Reina J Canales-Vaacutezquez E R
Losilla Journal of Power Sources 245 (2014) 107
67 S Nakayama T Kageyama H Aono Y Sadaoka J Mater Chem 5 (1995) 1801
68 Y Masubuchi M Higuchi T Takeda S Kikkawa J Alloys and Compounds 408
(2006) 641
69 J McFarlane S Barth M Swaffer J E H Sansom P R Slater Ionics 8 (2002)
149
70 S Tao J T S Irvine Mater ResBullet 36 (2001) 1245
71 J E H Sansom J R Tolchard P R Slater M S Islam Solid State Ionics 167
(2004) 17
Садржај Метода таложне титрације коришћена је за добијање чистог и допираног
хидроксиапатита јонима Sr Mg и Na (CaP CaMgP and CaSrNaP) За синтезу
супституисаних хидроксиапатитских материјала коришћена је модификована
таложна метода у односу на досада познате методе Због боље растворљивости и
знатно веће исплативости коришћени су раствори ацетата уместо нитрата
Синтетисани материјали су калцинисани на 400 degC а затим окарактерисани
рендгенском дифракцијом инфра црвеном спектроскопијом са Фуријеовом
трансформацијом и скенирајућом електронском микроскопијом Калцинисани узорци
су синтеровани на 1000 degC са задржавањем од 3 сата у атмосфери ваздуха (CaP1000
CaMgP1000 and CaSrNaP1000) Синтеровани материјали окарактерисани су рендгенском
дифракцијом на поликристалном узорку инфрацрвеном спекторскопијом са
Фуријеовом трансформацијом и скенирајућом електронском микроскопијом са
M Mirković et alScience of Sintering 50 (2018) 95-109 ___________________________________________________________________________
109
енергетско дисперзивном анализом као и методе комплексне импендансе Највећа
проводљивост добијена је на температури од 700 degC и износи 190times10-3
Ω-1
cm-1
за
мешовито допирани фосфатни материјал (CaSrNaP1000) Добијене активационе
енергије проводљивости износе 031 eV у температурном опсегу од 500-700 ordmC
Кључне речи Синтеровање Рендгенска дифракција Електрична проводљивост
Фосфатни материјали Горивне ћелије
copy 2016 Authors Published by the International Institute for the Science of Sintering This
article is an open access article distributed under the terms and conditions of the Creative
Commons mdash Attribution 40 International license
(httpscreativecommonsorglicensesby40)
M Mirković et alScience of Sintering 50 (2018) 95-109 ___________________________________________________________________________
103
became larger while small pores shrink and disappear [47 48] SEM micrograph of
CaMgP1000 (Fig 6b) shows numerous agglomerates of hexagonal particles with plate like
morphology bonded together with average grain size between 2 and 5 microm with non-uniform
particles with many vacancies less compact particle packing higher inner activity To
confirm the chemical composition the EDS microanalysis was done on the CaP1000
CaMgP1000 and CaSrNaP1000 sintered samples (Fig 6) It revealed that chemical composition
very similar to nominal chemical composition According to semi quantitative chemical
analysis the mean values for the ratios CaPO for CaP1000 sample determined from four runs
are 21275 These ratios correspond to empirically calculated compound with nominal
composition-(Ca21P2O75) The mean value of the chemical ratios CaMgPO for CaMgP1000
was also determined from four runs and amounts to 120727 which corresponds to
empirical compound with following composition-(Ca12Mg07)19P2O7 The mean value of the
chemical ratios Ca Sr Na PO of the NaSrHA1000 sample was determined from four runs
and amounts to 0506227 which corresponds to-Ca05Sr06Na2P2O7 These results (Fig 6)
confirm the assumption that sintered samples based on hydroxyapatite at high temperature
(1000 ordmC) remain sTab and almost nominal chemical composition The obtained EDS results
are in good agreement with XRD results
32 Electrical conductivity
Recently ionic conductors with the apatite type structures attracted a great deal of
attention as an alternative electrolytes to the conventional yttria-stabilized zirconia to operate
in ITndashSOFCs in the intermediate temperature range (600-800 ordmC) [49-52] Namely
hydroxyapatite belongs to apatitic group of phosphate minerals with general formula
Me10(XO4)6Y2 with ionic substitution ability [53] Based on XRD (Tab I) unit cell
parameters of CaMgP samples are slightly increased compared to pure CaP sample the
contraction of a and c parameter can be attributed to much smaller ionic radius of Mg2+
than
Ca2+
[54] The change of unit cell parameters a and c of CaSrNaP indicate that Sr2+
and Na+
ions are incorporated in structure and not just adhered to the crystal where Sr2+
will firstly
replace Ca2+
ion in Ca2 site [55] On the other hand Na+ ion as a smaller than Ca
2+ could
possible replace Ca2+
in Ca1 site while a vacancy may formed at the OH- site [56] At 1000degC
structural transformation of hydroxyapatite occurs and it transforms to a high temperature
mixed phosphate ceramics with M2X2O7 structure where X ions are tetrahedral coordinated
(Fig 7)
Fig 7 Structures of M2X2O7 a) αndashSr2P2O7 b) βndashCa2P2O7 and c) Na4P2O7 (the green spheres
are Sr atoms blue spheres are Ca atoms the yellow spheres are Na atoms the red spheres are
O atoms and the purple spheres are P atoms)
The crystal structures of calcium strontium and sodium phosphate materials are
comprised of diphosphate ions which are linked to metallic ions to form a three
M Mirković et al Science of Sintering 50 (2018) 95-109
___________________________________________________________________________
104
dimensional network in β ndash Ca2P2O7 α-Sr2P2O7 and Na4P2O7 structures [57 58] A
common characteristic of thermal behavior of all phosphates is that under thermal
treatment they undergo dehydratation-condensation reaction which leads to phosphate
tetrahedral bonded to mutual oxygen atom The ion conductivity in these materials
occurs mainly via O- interstitials with preferential cndashaxis conduction [50 51 56 59]
in comparison to O2-
vacancy migration in fluorite and perovskitendashbased electrolytes
[59-63] Generally the impedance spectra are presented as negative of imaginary component
of impedance (ndashZimag) versus real component of impedance (Zreal) ie as Nyquist plot The
semicircles at high and intermediate frequency are ascribed to bulk and grain boundary
processes respectively while semicircle at low frequency represents the electrode process
contribution [56] In our case for the potential application in IT-SOFCs the measurements of
ionic conductivity of electrolytes in solid state of the CaMgP1000 and CaSrNaP1000 samples
were done in intermediate temperature range of 500-700 ordmC with the increments of 50 ordmC
The original Nyquist plots recorded in the available frequency range (1 Hz-100 kHz) are
presented in Fig 8 As it could be seen (Fig 8a and 8b) with increasing the temperature the
values of both resistance elements (Rb and Rgb) obviously decrease which causes an increase
in ωmax Consequently the whole region of the impedance points shifts towards the lowndash
frequency semicircle and at higher temperatures instead of Rb and Rgb separately only the
whole sum Rb+Rgb became readable in the available frequency range In this case at higher
temperatures the time constants associated with the bulk and grain boundary impedances are
much lower than those associated with the electrode interface As a result semicircles due to
bulk and grain boundary disappear and only a single semicircle due to electrode interfacial
processes can be observed [64 66] Therefore only the whole sum Rb+Rig became readable
and the values of total resistance were estimated from the experimental cross section of
obtained semicircles with the real component of impedance (Zreal) this intercept is marked by
arrows in Fig 8 (inset) New semicircles are observed to appear in a lowndashfrequency region
being particularly visible in temperature range at 650-700 ordmC (Fig 8b) Almost doubtless it
originates from the oxygen electrode reactions O2O2-
[64] which does not belong to the
scope of this study
20 30 40 50 60 700
10
20
30
40
50
60
70 a)
20 30 40 50 60 700
1
2
3
4
5
6
7
8
9
10x103
x103
-Zim
ag
(c
m)
Zreal (cm)
x102
x102
500 C
550 C
600 C
-Zim
ag
(c
m)
Zreal (cm)
6 8 10 12 14 160
2
4
6
8
10
12
14
16 b)x102
6 8 10 12 14 160
2
x102
x102
-Zim
ag
(c
m)
Zreal (cm)
x102
650 C
700 C
-Zim
ag
(c
m)
Zreal (cm)
Fig 8 (a)(b) Complex impedance plots of the sintered samples CaSrNaP1000 measured in the
temperature range from 500 to 700 ordmC in air atmosphere The arrows indicate the points on
the real axis corresponding to the readings Rb+Rgb
The total ionic conductivity of the sintered CaSrNaP1000 sample is shown in Tab II Obtained
values (at 700 ordmC amount to 418times10-3
Ω-1
cm-1
) are slightly higher than previously published
ones [64 66] More specifically values of ionic conductivity observed at 700 ordmC in this work
M Mirković et alScience of Sintering 50 (2018) 95-109 ___________________________________________________________________________
105
are similar to values obtained at 800 and 900 ordmC [67-71] This holds also if we compare the
results we found at lower temperatures with the literature data [64-67]
Tab II The temperature dependence of total ionic conductivity (κ) of the sintered CaMgP1000
and CaSrNaP1000 sample
COMPOSITION
κ
(Ω-1
cm-1
)
500 ordmC
κ
(Ω-1
cm-1
)
550 ordmC
κ
(Ω-1
cm-1
)
600 ordmC
κ
(Ω-1
cm-1
)
650 ordmC
κ
(Ω-1
cm-1
)
700 ordmC
CaMgP1000 123times10-4
222times10-4
358times10-4
294times10-4
312times10-4
CaSrNaP1000 227times10-4
356times10-4
657times10-4
117times10-3
190times10-3
According to the results listed in Tab II the dependence log κ = f(1T) of the sintered
sample CaSrNaP1000 is presented in Fig 9 The activation energy of conductivity (Ea) was
calculated from the Arrhenius plot according to the equation
Tk
EAT a 1
lnln (1)
where σ is the conductivity T is the absolute temperature A is the pre-exponential factor and
k is the Boltzmann constant Activation energy of total conductivity for sintered CaSrNaP1000
sample is 031 eV Ea values presented in our work are much lower than previously found [67
68 70 71] In addition comparing the obtained values of Ea with activation energies of
similar oxygen ion conductors containing dopants [71] obtained Ea values were significantly
lower We suggest that this is a consequence of wellndashordered structure and better process
ability of nanopowders obtained application method
100 105 110 115 120 125 130-38
-36
-34
-32
-30
-28
-26
Ea = 031 eV
log
k
1000T (K-1)
Fig 9 The dependence log κ = f(1T) of the sintered CaSrNaP1000 sample
M Mirković et al Science of Sintering 50 (2018) 95-109
___________________________________________________________________________
106
4 Conclusions
The pure Ca and doped with Mg and NaSr hydroxyapatite (CaP CaMgP and NaSrP)
were successfully synthesized using slightly modified solution precipitation method (partial
substitution nitrates by acetate solutions) and thus we achieved higher solubility and more
cost effective synthesis Any secondary phase that formed during synthesis decomposed
during thermal treatment leaving pure CaP CaMgP and CaSrNaP hexagonal (P63m)
Ca5(PO4)3OH materials at room temperature During sintering process at 1000 ordmC in an air for
3 h we developed high temperature doped calcium phosphate materials ie CaMgP1000 and
CaSrNaP1000 materials SEM analysis showed that CaMgP1000 and CaSrNaP1000 materials have
high structural arrangement EDS analysis confirmed that we obtained phosphate compound
type-M2X2O7 The fine crystalline powders after sintering at 1000 ordmC achieved 98 of
theoretical density The maximum value of electrical conductivity at 700 ordmC was achieved for
CaSrNaP1000 material with amounts to 190times10-3
Ω-1
cm-1
Electrical characteristics of the
sintered samples indicate the superiority of solid solution for the application in SOFCs
Additionally considering high sintering temperature one can conclude that the obtained
phosphate material demonstrates high thermal stability an important characteristics in its
potential use as solid electrolyte However it is important to note that the future research
should include furthering lower temperature synthesis routes improving stability ITndashSOFCs
electrolyteelectrode molecular dynamics (MD) simulations of conduction mechanisms and
the general search for improved ion conductors Also the breadth of possible doping regimes
in this phosphate material provides new opportunities to design and optimize their transport
parameters for potential clean energy applications
Acknowledgements
This work was supported by the Ministry of Education Science and Technological
development of Serbia as part of Projects III 45012 and OI 176016
5 References
1 A V Galvagno Chiodo F Urbani F Freni Int J Hydrogen Energy 38 (2013)
3913
2 Y Takagi K Kerman C Ko S Ramanathan J Power Sources 243 (2013) 1
3 H J Alves C Bley Junior R R Niklevicz E P Frigo M S Frigo C H Coimbra-
Arauacutejo Int J Hydrogen Energy 38 (2013) 5215
4 J-J Liu S-Q Zhang W-D Wang J-F Gao W Liu C-S Chen J Power Sources
287 (2012) 287
5 J L Hertz H L Tuller J Electroceram 13 (2004) 663
6 S J Litzelman J L Hertz W Jung H L Tuller Fuel Cells 8 (2008) 294
7 EY Pikalova A A Murashkina V I Maragou A K Demin V N Strekalovsky
P E Tsiakaras Int Journal Hydrogen Energy 36 (2011) 6175
8 D Magdalena Int J Electrochem Sc 7 (2012) 2874
9 M Stojmenović S Bošković M Žunić B Babić B Matović D Bajuk-
Bogdanović Mat Chem Phys 153 (2015) 422
10 M Stojmenović M Žunić J Gulicovski D Bajuk-Bogdanović I Holclajtner-
Antunović V Dodevski J Mater Sci 50 (2015) 3781
11 A Laghzizil N Elherch A Bouhaouss G Lorente T Coradin J Livage Mater
Res Bull 36 (2001) 953
M Mirković et alScience of Sintering 50 (2018) 95-109 ___________________________________________________________________________
107
12 K Yamashita H Owada T Umegaki T Kanazawa T Futagami Solid State Ionics
28 (1988) 660
13 F Bakan O Laccedilin H Sarac Powder Technol 233 (2013) 295
14 M Sadat-Shojai M-T Khorasani A Jamshidi J Cryst Growth 361 (2012) 73
15 S Pramanik A K Agarwal K N Rai A Garg Ceram Int 33 (2007) 419
16 I Mobasherpour MS Heshajin A Kazemzadeh M Zakeri J Alloys Comp 430
(2007) 330
17 A K Nayak Int Jour ChemTech Res 2 (2010) 903
18 T T Trang Pham T Phuong Nguyen T Nam Pham T Phuong Vu D L Tran
HThai Vietnam Academy of Science and Technology 4 (2013) 1
19 S C Cox P Jamshidi L M Grover K K Mallick Mat Sci Eng C-Bio S 35
(2014) 106
20 E Champion Acta Biomater 9 (2013) 5855
21 S J Kalita A Bhardwaj H A Bhatt Mat Sci Eng C-Bio S 27 (2007) 441
22 S Ramesh K L Aw R Tolouei M Amiriyan C Y Tan M Hamdi Ceram Int
39 (2013) 111
23 Đ Veljović G Vuković I Steins E Palcevskis P S Usković R Petrović Sci
Sinter 45 (2013) 233
24 S Raynaud E Champion D Bernache-Assollant Biomater 23 (2002) 1073
25 Rigaku Corporation Tokyo J PDXL Version 2030 Integrated X-ray Powder
Diffraction Software 2011
26 Powder Diffraction File P-D announcement of new data-base release 2012
International Centrefor Diffraction Data (ICDD)
27 K Momma F Izumi J Appl Crystallogr 44 (2011) 1272
28 M Prekajski M Mirković B Todorović A Matković M Marinović-Cincović J
Luković B Matović J Eur CeramSoc 36 (2016) 1293
29 J Wang LL Shaw Adv Mater 19 (2007) 2364
30 A Mortier J Lemaitre P Rouxet Thermochim Acta 113 (1987) 133
31 R T Downs M Hall-Wallace Am Mineralog 88 (2003) 247
32 G Qi S Zhang K A Khor C Liu X Zeng W Weng Thin Solid Films 516
(2008) 5176
33 S C Cox P Jamshidi L M Grover K K Mallick Mat Sci Eng C-Bio S 35
(2014) 106
34 A Cuccu S Montinaro R Orrugrave G Cao D Bellucci A Sola Ceram Int 41
(2015)725
35 W Zhang Y Shen H Pan K Lin X Liu B W Darvell Acta Biomater 7 (2011)
800
36 J A Gadsden Infrared spectra of minerals and related inorganic compounds
Butterworths London 1975
37 M Iafisco A Ruffini A Adamiano S Sprio A Tampieri Mat Sci Eng C-Bio S
35 (2014) 212
38 S Koutsopoulos J Biomed Mater Res 62 (2002) 600
39 S-C Liou S-Y Chen H-Y Lee J-S Bow Biomaterials 25 (2004) 189
40 M E Fleet Biomaterials 30 (2009) 1473
41 N V Chukanov Infrared spectra of mineral species Extended library Springer
London Limited 2013
42 V Koleva V Stefov M Najdoski A Cahil Thermochim Acta 619 (2015) 20
43 B Mehdikhani G H Borhani J Ceram Process Res 16 (2015) 308
44 P Solanki S Vasant M Joshi Int J Appl Ceram Tech 11 (2014) 663
45 G B Balazs R Glass Solid State Ionics 76 (1995) 155
46 G Buvaneswari K Valsalan Mater Lett 134 (2014) 252
M Mirković et al Science of Sintering 50 (2018) 95-109
___________________________________________________________________________
108
47 A Farzadi F Bakhshi M Solati-Hashjin M Asadi-Eydivand N A Osman Ceram
Int40 (2014) 6021
48 A G Evans C H Hsueh J Am Ceram Soc 69 (1986) 444
49 A J Jacobson Chem Mater 22 (2010) 660
50 A Orera P R Slater Chem Mater 22 (2010) 675
51 L Malavasi C A J Fisher M S Islam Chem Soc Review 39 (2010) 4370
52 D Marrero-Loacutepez M C Martiacuten-Sedentildeo J Pentildea-Martiacutenez J C Ruiz-Morales P
Nuacutentildeez M A G Aranda J Power Sources 195 (2010) 2496
53 M P Moreira G D de Almeida Soares J Dentzer K Anselme L Aacute de Sena A
Kuznetsov Mat Sci Eng C-Bio S 61 (2016) 736
54 F Ren Y Leng R Xin X Ge Acta Biomater 6 (2010) 2787
55 Z Y Li W M Lam C Yang B Xu G X Ni S A Abbah Biomaterials 28
(2007)1452
56 Z Leilei L Hejun L Kezhi F Qiangang Z Yulei L Shoujie Ceram Int 40
(2014) 13123
57 K Y Leung C Calvo Can J Chem 50 (1972) 2519
58 H Larsove J Ingela M Christina Acta Chemica Scandinavica 22 (1968) 1419
59 E Kendrick M S Islam P R Slater J Mater Chem 17 (2007) 3104
60 E Kendrick M S Islam P R Slater Chem Commun 715 (2008) 7
61 K Fukuda T Asaka R Hamaguchi T Suzuki H Oka A Berghout Chem Mater
23 (2011) 5474
62 E Kendrick M S Islam P R Slater Solid State Ionics 177 (2007) 3411
63 L Leon-Reina E R Losilla M Martinez-Lara S Bruque M A G J Mater
Chem 14 (2004) 1142
64 N Nallamuthu I Prakash N Satyanarayana M Venkateswarlu J Alloys and
Compounds 509 (2011) 1138
65 M Sato Y Kono H Ueda K Uematsu K Toda Solid State Ionics 83 (1996)249
66 D Marrero-Loacutepez L dos Santos-Goacutemez L Leoacuten-Reina J Canales-Vaacutezquez E R
Losilla Journal of Power Sources 245 (2014) 107
67 S Nakayama T Kageyama H Aono Y Sadaoka J Mater Chem 5 (1995) 1801
68 Y Masubuchi M Higuchi T Takeda S Kikkawa J Alloys and Compounds 408
(2006) 641
69 J McFarlane S Barth M Swaffer J E H Sansom P R Slater Ionics 8 (2002)
149
70 S Tao J T S Irvine Mater ResBullet 36 (2001) 1245
71 J E H Sansom J R Tolchard P R Slater M S Islam Solid State Ionics 167
(2004) 17
Садржај Метода таложне титрације коришћена је за добијање чистог и допираног
хидроксиапатита јонима Sr Mg и Na (CaP CaMgP and CaSrNaP) За синтезу
супституисаних хидроксиапатитских материјала коришћена је модификована
таложна метода у односу на досада познате методе Због боље растворљивости и
знатно веће исплативости коришћени су раствори ацетата уместо нитрата
Синтетисани материјали су калцинисани на 400 degC а затим окарактерисани
рендгенском дифракцијом инфра црвеном спектроскопијом са Фуријеовом
трансформацијом и скенирајућом електронском микроскопијом Калцинисани узорци
су синтеровани на 1000 degC са задржавањем од 3 сата у атмосфери ваздуха (CaP1000
CaMgP1000 and CaSrNaP1000) Синтеровани материјали окарактерисани су рендгенском
дифракцијом на поликристалном узорку инфрацрвеном спекторскопијом са
Фуријеовом трансформацијом и скенирајућом електронском микроскопијом са
M Mirković et alScience of Sintering 50 (2018) 95-109 ___________________________________________________________________________
109
енергетско дисперзивном анализом као и методе комплексне импендансе Највећа
проводљивост добијена је на температури од 700 degC и износи 190times10-3
Ω-1
cm-1
за
мешовито допирани фосфатни материјал (CaSrNaP1000) Добијене активационе
енергије проводљивости износе 031 eV у температурном опсегу од 500-700 ordmC
Кључне речи Синтеровање Рендгенска дифракција Електрична проводљивост
Фосфатни материјали Горивне ћелије
copy 2016 Authors Published by the International Institute for the Science of Sintering This
article is an open access article distributed under the terms and conditions of the Creative
Commons mdash Attribution 40 International license
(httpscreativecommonsorglicensesby40)
M Mirković et al Science of Sintering 50 (2018) 95-109
___________________________________________________________________________
104
dimensional network in β ndash Ca2P2O7 α-Sr2P2O7 and Na4P2O7 structures [57 58] A
common characteristic of thermal behavior of all phosphates is that under thermal
treatment they undergo dehydratation-condensation reaction which leads to phosphate
tetrahedral bonded to mutual oxygen atom The ion conductivity in these materials
occurs mainly via O- interstitials with preferential cndashaxis conduction [50 51 56 59]
in comparison to O2-
vacancy migration in fluorite and perovskitendashbased electrolytes
[59-63] Generally the impedance spectra are presented as negative of imaginary component
of impedance (ndashZimag) versus real component of impedance (Zreal) ie as Nyquist plot The
semicircles at high and intermediate frequency are ascribed to bulk and grain boundary
processes respectively while semicircle at low frequency represents the electrode process
contribution [56] In our case for the potential application in IT-SOFCs the measurements of
ionic conductivity of electrolytes in solid state of the CaMgP1000 and CaSrNaP1000 samples
were done in intermediate temperature range of 500-700 ordmC with the increments of 50 ordmC
The original Nyquist plots recorded in the available frequency range (1 Hz-100 kHz) are
presented in Fig 8 As it could be seen (Fig 8a and 8b) with increasing the temperature the
values of both resistance elements (Rb and Rgb) obviously decrease which causes an increase
in ωmax Consequently the whole region of the impedance points shifts towards the lowndash
frequency semicircle and at higher temperatures instead of Rb and Rgb separately only the
whole sum Rb+Rgb became readable in the available frequency range In this case at higher
temperatures the time constants associated with the bulk and grain boundary impedances are
much lower than those associated with the electrode interface As a result semicircles due to
bulk and grain boundary disappear and only a single semicircle due to electrode interfacial
processes can be observed [64 66] Therefore only the whole sum Rb+Rig became readable
and the values of total resistance were estimated from the experimental cross section of
obtained semicircles with the real component of impedance (Zreal) this intercept is marked by
arrows in Fig 8 (inset) New semicircles are observed to appear in a lowndashfrequency region
being particularly visible in temperature range at 650-700 ordmC (Fig 8b) Almost doubtless it
originates from the oxygen electrode reactions O2O2-
[64] which does not belong to the
scope of this study
20 30 40 50 60 700
10
20
30
40
50
60
70 a)
20 30 40 50 60 700
1
2
3
4
5
6
7
8
9
10x103
x103
-Zim
ag
(c
m)
Zreal (cm)
x102
x102
500 C
550 C
600 C
-Zim
ag
(c
m)
Zreal (cm)
6 8 10 12 14 160
2
4
6
8
10
12
14
16 b)x102
6 8 10 12 14 160
2
x102
x102
-Zim
ag
(c
m)
Zreal (cm)
x102
650 C
700 C
-Zim
ag
(c
m)
Zreal (cm)
Fig 8 (a)(b) Complex impedance plots of the sintered samples CaSrNaP1000 measured in the
temperature range from 500 to 700 ordmC in air atmosphere The arrows indicate the points on
the real axis corresponding to the readings Rb+Rgb
The total ionic conductivity of the sintered CaSrNaP1000 sample is shown in Tab II Obtained
values (at 700 ordmC amount to 418times10-3
Ω-1
cm-1
) are slightly higher than previously published
ones [64 66] More specifically values of ionic conductivity observed at 700 ordmC in this work
M Mirković et alScience of Sintering 50 (2018) 95-109 ___________________________________________________________________________
105
are similar to values obtained at 800 and 900 ordmC [67-71] This holds also if we compare the
results we found at lower temperatures with the literature data [64-67]
Tab II The temperature dependence of total ionic conductivity (κ) of the sintered CaMgP1000
and CaSrNaP1000 sample
COMPOSITION
κ
(Ω-1
cm-1
)
500 ordmC
κ
(Ω-1
cm-1
)
550 ordmC
κ
(Ω-1
cm-1
)
600 ordmC
κ
(Ω-1
cm-1
)
650 ordmC
κ
(Ω-1
cm-1
)
700 ordmC
CaMgP1000 123times10-4
222times10-4
358times10-4
294times10-4
312times10-4
CaSrNaP1000 227times10-4
356times10-4
657times10-4
117times10-3
190times10-3
According to the results listed in Tab II the dependence log κ = f(1T) of the sintered
sample CaSrNaP1000 is presented in Fig 9 The activation energy of conductivity (Ea) was
calculated from the Arrhenius plot according to the equation
Tk
EAT a 1
lnln (1)
where σ is the conductivity T is the absolute temperature A is the pre-exponential factor and
k is the Boltzmann constant Activation energy of total conductivity for sintered CaSrNaP1000
sample is 031 eV Ea values presented in our work are much lower than previously found [67
68 70 71] In addition comparing the obtained values of Ea with activation energies of
similar oxygen ion conductors containing dopants [71] obtained Ea values were significantly
lower We suggest that this is a consequence of wellndashordered structure and better process
ability of nanopowders obtained application method
100 105 110 115 120 125 130-38
-36
-34
-32
-30
-28
-26
Ea = 031 eV
log
k
1000T (K-1)
Fig 9 The dependence log κ = f(1T) of the sintered CaSrNaP1000 sample
M Mirković et al Science of Sintering 50 (2018) 95-109
___________________________________________________________________________
106
4 Conclusions
The pure Ca and doped with Mg and NaSr hydroxyapatite (CaP CaMgP and NaSrP)
were successfully synthesized using slightly modified solution precipitation method (partial
substitution nitrates by acetate solutions) and thus we achieved higher solubility and more
cost effective synthesis Any secondary phase that formed during synthesis decomposed
during thermal treatment leaving pure CaP CaMgP and CaSrNaP hexagonal (P63m)
Ca5(PO4)3OH materials at room temperature During sintering process at 1000 ordmC in an air for
3 h we developed high temperature doped calcium phosphate materials ie CaMgP1000 and
CaSrNaP1000 materials SEM analysis showed that CaMgP1000 and CaSrNaP1000 materials have
high structural arrangement EDS analysis confirmed that we obtained phosphate compound
type-M2X2O7 The fine crystalline powders after sintering at 1000 ordmC achieved 98 of
theoretical density The maximum value of electrical conductivity at 700 ordmC was achieved for
CaSrNaP1000 material with amounts to 190times10-3
Ω-1
cm-1
Electrical characteristics of the
sintered samples indicate the superiority of solid solution for the application in SOFCs
Additionally considering high sintering temperature one can conclude that the obtained
phosphate material demonstrates high thermal stability an important characteristics in its
potential use as solid electrolyte However it is important to note that the future research
should include furthering lower temperature synthesis routes improving stability ITndashSOFCs
electrolyteelectrode molecular dynamics (MD) simulations of conduction mechanisms and
the general search for improved ion conductors Also the breadth of possible doping regimes
in this phosphate material provides new opportunities to design and optimize their transport
parameters for potential clean energy applications
Acknowledgements
This work was supported by the Ministry of Education Science and Technological
development of Serbia as part of Projects III 45012 and OI 176016
5 References
1 A V Galvagno Chiodo F Urbani F Freni Int J Hydrogen Energy 38 (2013)
3913
2 Y Takagi K Kerman C Ko S Ramanathan J Power Sources 243 (2013) 1
3 H J Alves C Bley Junior R R Niklevicz E P Frigo M S Frigo C H Coimbra-
Arauacutejo Int J Hydrogen Energy 38 (2013) 5215
4 J-J Liu S-Q Zhang W-D Wang J-F Gao W Liu C-S Chen J Power Sources
287 (2012) 287
5 J L Hertz H L Tuller J Electroceram 13 (2004) 663
6 S J Litzelman J L Hertz W Jung H L Tuller Fuel Cells 8 (2008) 294
7 EY Pikalova A A Murashkina V I Maragou A K Demin V N Strekalovsky
P E Tsiakaras Int Journal Hydrogen Energy 36 (2011) 6175
8 D Magdalena Int J Electrochem Sc 7 (2012) 2874
9 M Stojmenović S Bošković M Žunić B Babić B Matović D Bajuk-
Bogdanović Mat Chem Phys 153 (2015) 422
10 M Stojmenović M Žunić J Gulicovski D Bajuk-Bogdanović I Holclajtner-
Antunović V Dodevski J Mater Sci 50 (2015) 3781
11 A Laghzizil N Elherch A Bouhaouss G Lorente T Coradin J Livage Mater
Res Bull 36 (2001) 953
M Mirković et alScience of Sintering 50 (2018) 95-109 ___________________________________________________________________________
107
12 K Yamashita H Owada T Umegaki T Kanazawa T Futagami Solid State Ionics
28 (1988) 660
13 F Bakan O Laccedilin H Sarac Powder Technol 233 (2013) 295
14 M Sadat-Shojai M-T Khorasani A Jamshidi J Cryst Growth 361 (2012) 73
15 S Pramanik A K Agarwal K N Rai A Garg Ceram Int 33 (2007) 419
16 I Mobasherpour MS Heshajin A Kazemzadeh M Zakeri J Alloys Comp 430
(2007) 330
17 A K Nayak Int Jour ChemTech Res 2 (2010) 903
18 T T Trang Pham T Phuong Nguyen T Nam Pham T Phuong Vu D L Tran
HThai Vietnam Academy of Science and Technology 4 (2013) 1
19 S C Cox P Jamshidi L M Grover K K Mallick Mat Sci Eng C-Bio S 35
(2014) 106
20 E Champion Acta Biomater 9 (2013) 5855
21 S J Kalita A Bhardwaj H A Bhatt Mat Sci Eng C-Bio S 27 (2007) 441
22 S Ramesh K L Aw R Tolouei M Amiriyan C Y Tan M Hamdi Ceram Int
39 (2013) 111
23 Đ Veljović G Vuković I Steins E Palcevskis P S Usković R Petrović Sci
Sinter 45 (2013) 233
24 S Raynaud E Champion D Bernache-Assollant Biomater 23 (2002) 1073
25 Rigaku Corporation Tokyo J PDXL Version 2030 Integrated X-ray Powder
Diffraction Software 2011
26 Powder Diffraction File P-D announcement of new data-base release 2012
International Centrefor Diffraction Data (ICDD)
27 K Momma F Izumi J Appl Crystallogr 44 (2011) 1272
28 M Prekajski M Mirković B Todorović A Matković M Marinović-Cincović J
Luković B Matović J Eur CeramSoc 36 (2016) 1293
29 J Wang LL Shaw Adv Mater 19 (2007) 2364
30 A Mortier J Lemaitre P Rouxet Thermochim Acta 113 (1987) 133
31 R T Downs M Hall-Wallace Am Mineralog 88 (2003) 247
32 G Qi S Zhang K A Khor C Liu X Zeng W Weng Thin Solid Films 516
(2008) 5176
33 S C Cox P Jamshidi L M Grover K K Mallick Mat Sci Eng C-Bio S 35
(2014) 106
34 A Cuccu S Montinaro R Orrugrave G Cao D Bellucci A Sola Ceram Int 41
(2015)725
35 W Zhang Y Shen H Pan K Lin X Liu B W Darvell Acta Biomater 7 (2011)
800
36 J A Gadsden Infrared spectra of minerals and related inorganic compounds
Butterworths London 1975
37 M Iafisco A Ruffini A Adamiano S Sprio A Tampieri Mat Sci Eng C-Bio S
35 (2014) 212
38 S Koutsopoulos J Biomed Mater Res 62 (2002) 600
39 S-C Liou S-Y Chen H-Y Lee J-S Bow Biomaterials 25 (2004) 189
40 M E Fleet Biomaterials 30 (2009) 1473
41 N V Chukanov Infrared spectra of mineral species Extended library Springer
London Limited 2013
42 V Koleva V Stefov M Najdoski A Cahil Thermochim Acta 619 (2015) 20
43 B Mehdikhani G H Borhani J Ceram Process Res 16 (2015) 308
44 P Solanki S Vasant M Joshi Int J Appl Ceram Tech 11 (2014) 663
45 G B Balazs R Glass Solid State Ionics 76 (1995) 155
46 G Buvaneswari K Valsalan Mater Lett 134 (2014) 252
M Mirković et al Science of Sintering 50 (2018) 95-109
___________________________________________________________________________
108
47 A Farzadi F Bakhshi M Solati-Hashjin M Asadi-Eydivand N A Osman Ceram
Int40 (2014) 6021
48 A G Evans C H Hsueh J Am Ceram Soc 69 (1986) 444
49 A J Jacobson Chem Mater 22 (2010) 660
50 A Orera P R Slater Chem Mater 22 (2010) 675
51 L Malavasi C A J Fisher M S Islam Chem Soc Review 39 (2010) 4370
52 D Marrero-Loacutepez M C Martiacuten-Sedentildeo J Pentildea-Martiacutenez J C Ruiz-Morales P
Nuacutentildeez M A G Aranda J Power Sources 195 (2010) 2496
53 M P Moreira G D de Almeida Soares J Dentzer K Anselme L Aacute de Sena A
Kuznetsov Mat Sci Eng C-Bio S 61 (2016) 736
54 F Ren Y Leng R Xin X Ge Acta Biomater 6 (2010) 2787
55 Z Y Li W M Lam C Yang B Xu G X Ni S A Abbah Biomaterials 28
(2007)1452
56 Z Leilei L Hejun L Kezhi F Qiangang Z Yulei L Shoujie Ceram Int 40
(2014) 13123
57 K Y Leung C Calvo Can J Chem 50 (1972) 2519
58 H Larsove J Ingela M Christina Acta Chemica Scandinavica 22 (1968) 1419
59 E Kendrick M S Islam P R Slater J Mater Chem 17 (2007) 3104
60 E Kendrick M S Islam P R Slater Chem Commun 715 (2008) 7
61 K Fukuda T Asaka R Hamaguchi T Suzuki H Oka A Berghout Chem Mater
23 (2011) 5474
62 E Kendrick M S Islam P R Slater Solid State Ionics 177 (2007) 3411
63 L Leon-Reina E R Losilla M Martinez-Lara S Bruque M A G J Mater
Chem 14 (2004) 1142
64 N Nallamuthu I Prakash N Satyanarayana M Venkateswarlu J Alloys and
Compounds 509 (2011) 1138
65 M Sato Y Kono H Ueda K Uematsu K Toda Solid State Ionics 83 (1996)249
66 D Marrero-Loacutepez L dos Santos-Goacutemez L Leoacuten-Reina J Canales-Vaacutezquez E R
Losilla Journal of Power Sources 245 (2014) 107
67 S Nakayama T Kageyama H Aono Y Sadaoka J Mater Chem 5 (1995) 1801
68 Y Masubuchi M Higuchi T Takeda S Kikkawa J Alloys and Compounds 408
(2006) 641
69 J McFarlane S Barth M Swaffer J E H Sansom P R Slater Ionics 8 (2002)
149
70 S Tao J T S Irvine Mater ResBullet 36 (2001) 1245
71 J E H Sansom J R Tolchard P R Slater M S Islam Solid State Ionics 167
(2004) 17
Садржај Метода таложне титрације коришћена је за добијање чистог и допираног
хидроксиапатита јонима Sr Mg и Na (CaP CaMgP and CaSrNaP) За синтезу
супституисаних хидроксиапатитских материјала коришћена је модификована
таложна метода у односу на досада познате методе Због боље растворљивости и
знатно веће исплативости коришћени су раствори ацетата уместо нитрата
Синтетисани материјали су калцинисани на 400 degC а затим окарактерисани
рендгенском дифракцијом инфра црвеном спектроскопијом са Фуријеовом
трансформацијом и скенирајућом електронском микроскопијом Калцинисани узорци
су синтеровани на 1000 degC са задржавањем од 3 сата у атмосфери ваздуха (CaP1000
CaMgP1000 and CaSrNaP1000) Синтеровани материјали окарактерисани су рендгенском
дифракцијом на поликристалном узорку инфрацрвеном спекторскопијом са
Фуријеовом трансформацијом и скенирајућом електронском микроскопијом са
M Mirković et alScience of Sintering 50 (2018) 95-109 ___________________________________________________________________________
109
енергетско дисперзивном анализом као и методе комплексне импендансе Највећа
проводљивост добијена је на температури од 700 degC и износи 190times10-3
Ω-1
cm-1
за
мешовито допирани фосфатни материјал (CaSrNaP1000) Добијене активационе
енергије проводљивости износе 031 eV у температурном опсегу од 500-700 ordmC
Кључне речи Синтеровање Рендгенска дифракција Електрична проводљивост
Фосфатни материјали Горивне ћелије
copy 2016 Authors Published by the International Institute for the Science of Sintering This
article is an open access article distributed under the terms and conditions of the Creative
Commons mdash Attribution 40 International license
(httpscreativecommonsorglicensesby40)
M Mirković et alScience of Sintering 50 (2018) 95-109 ___________________________________________________________________________
105
are similar to values obtained at 800 and 900 ordmC [67-71] This holds also if we compare the
results we found at lower temperatures with the literature data [64-67]
Tab II The temperature dependence of total ionic conductivity (κ) of the sintered CaMgP1000
and CaSrNaP1000 sample
COMPOSITION
κ
(Ω-1
cm-1
)
500 ordmC
κ
(Ω-1
cm-1
)
550 ordmC
κ
(Ω-1
cm-1
)
600 ordmC
κ
(Ω-1
cm-1
)
650 ordmC
κ
(Ω-1
cm-1
)
700 ordmC
CaMgP1000 123times10-4
222times10-4
358times10-4
294times10-4
312times10-4
CaSrNaP1000 227times10-4
356times10-4
657times10-4
117times10-3
190times10-3
According to the results listed in Tab II the dependence log κ = f(1T) of the sintered
sample CaSrNaP1000 is presented in Fig 9 The activation energy of conductivity (Ea) was
calculated from the Arrhenius plot according to the equation
Tk
EAT a 1
lnln (1)
where σ is the conductivity T is the absolute temperature A is the pre-exponential factor and
k is the Boltzmann constant Activation energy of total conductivity for sintered CaSrNaP1000
sample is 031 eV Ea values presented in our work are much lower than previously found [67
68 70 71] In addition comparing the obtained values of Ea with activation energies of
similar oxygen ion conductors containing dopants [71] obtained Ea values were significantly
lower We suggest that this is a consequence of wellndashordered structure and better process
ability of nanopowders obtained application method
100 105 110 115 120 125 130-38
-36
-34
-32
-30
-28
-26
Ea = 031 eV
log
k
1000T (K-1)
Fig 9 The dependence log κ = f(1T) of the sintered CaSrNaP1000 sample
M Mirković et al Science of Sintering 50 (2018) 95-109
___________________________________________________________________________
106
4 Conclusions
The pure Ca and doped with Mg and NaSr hydroxyapatite (CaP CaMgP and NaSrP)
were successfully synthesized using slightly modified solution precipitation method (partial
substitution nitrates by acetate solutions) and thus we achieved higher solubility and more
cost effective synthesis Any secondary phase that formed during synthesis decomposed
during thermal treatment leaving pure CaP CaMgP and CaSrNaP hexagonal (P63m)
Ca5(PO4)3OH materials at room temperature During sintering process at 1000 ordmC in an air for
3 h we developed high temperature doped calcium phosphate materials ie CaMgP1000 and
CaSrNaP1000 materials SEM analysis showed that CaMgP1000 and CaSrNaP1000 materials have
high structural arrangement EDS analysis confirmed that we obtained phosphate compound
type-M2X2O7 The fine crystalline powders after sintering at 1000 ordmC achieved 98 of
theoretical density The maximum value of electrical conductivity at 700 ordmC was achieved for
CaSrNaP1000 material with amounts to 190times10-3
Ω-1
cm-1
Electrical characteristics of the
sintered samples indicate the superiority of solid solution for the application in SOFCs
Additionally considering high sintering temperature one can conclude that the obtained
phosphate material demonstrates high thermal stability an important characteristics in its
potential use as solid electrolyte However it is important to note that the future research
should include furthering lower temperature synthesis routes improving stability ITndashSOFCs
electrolyteelectrode molecular dynamics (MD) simulations of conduction mechanisms and
the general search for improved ion conductors Also the breadth of possible doping regimes
in this phosphate material provides new opportunities to design and optimize their transport
parameters for potential clean energy applications
Acknowledgements
This work was supported by the Ministry of Education Science and Technological
development of Serbia as part of Projects III 45012 and OI 176016
5 References
1 A V Galvagno Chiodo F Urbani F Freni Int J Hydrogen Energy 38 (2013)
3913
2 Y Takagi K Kerman C Ko S Ramanathan J Power Sources 243 (2013) 1
3 H J Alves C Bley Junior R R Niklevicz E P Frigo M S Frigo C H Coimbra-
Arauacutejo Int J Hydrogen Energy 38 (2013) 5215
4 J-J Liu S-Q Zhang W-D Wang J-F Gao W Liu C-S Chen J Power Sources
287 (2012) 287
5 J L Hertz H L Tuller J Electroceram 13 (2004) 663
6 S J Litzelman J L Hertz W Jung H L Tuller Fuel Cells 8 (2008) 294
7 EY Pikalova A A Murashkina V I Maragou A K Demin V N Strekalovsky
P E Tsiakaras Int Journal Hydrogen Energy 36 (2011) 6175
8 D Magdalena Int J Electrochem Sc 7 (2012) 2874
9 M Stojmenović S Bošković M Žunić B Babić B Matović D Bajuk-
Bogdanović Mat Chem Phys 153 (2015) 422
10 M Stojmenović M Žunić J Gulicovski D Bajuk-Bogdanović I Holclajtner-
Antunović V Dodevski J Mater Sci 50 (2015) 3781
11 A Laghzizil N Elherch A Bouhaouss G Lorente T Coradin J Livage Mater
Res Bull 36 (2001) 953
M Mirković et alScience of Sintering 50 (2018) 95-109 ___________________________________________________________________________
107
12 K Yamashita H Owada T Umegaki T Kanazawa T Futagami Solid State Ionics
28 (1988) 660
13 F Bakan O Laccedilin H Sarac Powder Technol 233 (2013) 295
14 M Sadat-Shojai M-T Khorasani A Jamshidi J Cryst Growth 361 (2012) 73
15 S Pramanik A K Agarwal K N Rai A Garg Ceram Int 33 (2007) 419
16 I Mobasherpour MS Heshajin A Kazemzadeh M Zakeri J Alloys Comp 430
(2007) 330
17 A K Nayak Int Jour ChemTech Res 2 (2010) 903
18 T T Trang Pham T Phuong Nguyen T Nam Pham T Phuong Vu D L Tran
HThai Vietnam Academy of Science and Technology 4 (2013) 1
19 S C Cox P Jamshidi L M Grover K K Mallick Mat Sci Eng C-Bio S 35
(2014) 106
20 E Champion Acta Biomater 9 (2013) 5855
21 S J Kalita A Bhardwaj H A Bhatt Mat Sci Eng C-Bio S 27 (2007) 441
22 S Ramesh K L Aw R Tolouei M Amiriyan C Y Tan M Hamdi Ceram Int
39 (2013) 111
23 Đ Veljović G Vuković I Steins E Palcevskis P S Usković R Petrović Sci
Sinter 45 (2013) 233
24 S Raynaud E Champion D Bernache-Assollant Biomater 23 (2002) 1073
25 Rigaku Corporation Tokyo J PDXL Version 2030 Integrated X-ray Powder
Diffraction Software 2011
26 Powder Diffraction File P-D announcement of new data-base release 2012
International Centrefor Diffraction Data (ICDD)
27 K Momma F Izumi J Appl Crystallogr 44 (2011) 1272
28 M Prekajski M Mirković B Todorović A Matković M Marinović-Cincović J
Luković B Matović J Eur CeramSoc 36 (2016) 1293
29 J Wang LL Shaw Adv Mater 19 (2007) 2364
30 A Mortier J Lemaitre P Rouxet Thermochim Acta 113 (1987) 133
31 R T Downs M Hall-Wallace Am Mineralog 88 (2003) 247
32 G Qi S Zhang K A Khor C Liu X Zeng W Weng Thin Solid Films 516
(2008) 5176
33 S C Cox P Jamshidi L M Grover K K Mallick Mat Sci Eng C-Bio S 35
(2014) 106
34 A Cuccu S Montinaro R Orrugrave G Cao D Bellucci A Sola Ceram Int 41
(2015)725
35 W Zhang Y Shen H Pan K Lin X Liu B W Darvell Acta Biomater 7 (2011)
800
36 J A Gadsden Infrared spectra of minerals and related inorganic compounds
Butterworths London 1975
37 M Iafisco A Ruffini A Adamiano S Sprio A Tampieri Mat Sci Eng C-Bio S
35 (2014) 212
38 S Koutsopoulos J Biomed Mater Res 62 (2002) 600
39 S-C Liou S-Y Chen H-Y Lee J-S Bow Biomaterials 25 (2004) 189
40 M E Fleet Biomaterials 30 (2009) 1473
41 N V Chukanov Infrared spectra of mineral species Extended library Springer
London Limited 2013
42 V Koleva V Stefov M Najdoski A Cahil Thermochim Acta 619 (2015) 20
43 B Mehdikhani G H Borhani J Ceram Process Res 16 (2015) 308
44 P Solanki S Vasant M Joshi Int J Appl Ceram Tech 11 (2014) 663
45 G B Balazs R Glass Solid State Ionics 76 (1995) 155
46 G Buvaneswari K Valsalan Mater Lett 134 (2014) 252
M Mirković et al Science of Sintering 50 (2018) 95-109
___________________________________________________________________________
108
47 A Farzadi F Bakhshi M Solati-Hashjin M Asadi-Eydivand N A Osman Ceram
Int40 (2014) 6021
48 A G Evans C H Hsueh J Am Ceram Soc 69 (1986) 444
49 A J Jacobson Chem Mater 22 (2010) 660
50 A Orera P R Slater Chem Mater 22 (2010) 675
51 L Malavasi C A J Fisher M S Islam Chem Soc Review 39 (2010) 4370
52 D Marrero-Loacutepez M C Martiacuten-Sedentildeo J Pentildea-Martiacutenez J C Ruiz-Morales P
Nuacutentildeez M A G Aranda J Power Sources 195 (2010) 2496
53 M P Moreira G D de Almeida Soares J Dentzer K Anselme L Aacute de Sena A
Kuznetsov Mat Sci Eng C-Bio S 61 (2016) 736
54 F Ren Y Leng R Xin X Ge Acta Biomater 6 (2010) 2787
55 Z Y Li W M Lam C Yang B Xu G X Ni S A Abbah Biomaterials 28
(2007)1452
56 Z Leilei L Hejun L Kezhi F Qiangang Z Yulei L Shoujie Ceram Int 40
(2014) 13123
57 K Y Leung C Calvo Can J Chem 50 (1972) 2519
58 H Larsove J Ingela M Christina Acta Chemica Scandinavica 22 (1968) 1419
59 E Kendrick M S Islam P R Slater J Mater Chem 17 (2007) 3104
60 E Kendrick M S Islam P R Slater Chem Commun 715 (2008) 7
61 K Fukuda T Asaka R Hamaguchi T Suzuki H Oka A Berghout Chem Mater
23 (2011) 5474
62 E Kendrick M S Islam P R Slater Solid State Ionics 177 (2007) 3411
63 L Leon-Reina E R Losilla M Martinez-Lara S Bruque M A G J Mater
Chem 14 (2004) 1142
64 N Nallamuthu I Prakash N Satyanarayana M Venkateswarlu J Alloys and
Compounds 509 (2011) 1138
65 M Sato Y Kono H Ueda K Uematsu K Toda Solid State Ionics 83 (1996)249
66 D Marrero-Loacutepez L dos Santos-Goacutemez L Leoacuten-Reina J Canales-Vaacutezquez E R
Losilla Journal of Power Sources 245 (2014) 107
67 S Nakayama T Kageyama H Aono Y Sadaoka J Mater Chem 5 (1995) 1801
68 Y Masubuchi M Higuchi T Takeda S Kikkawa J Alloys and Compounds 408
(2006) 641
69 J McFarlane S Barth M Swaffer J E H Sansom P R Slater Ionics 8 (2002)
149
70 S Tao J T S Irvine Mater ResBullet 36 (2001) 1245
71 J E H Sansom J R Tolchard P R Slater M S Islam Solid State Ionics 167
(2004) 17
Садржај Метода таложне титрације коришћена је за добијање чистог и допираног
хидроксиапатита јонима Sr Mg и Na (CaP CaMgP and CaSrNaP) За синтезу
супституисаних хидроксиапатитских материјала коришћена је модификована
таложна метода у односу на досада познате методе Због боље растворљивости и
знатно веће исплативости коришћени су раствори ацетата уместо нитрата
Синтетисани материјали су калцинисани на 400 degC а затим окарактерисани
рендгенском дифракцијом инфра црвеном спектроскопијом са Фуријеовом
трансформацијом и скенирајућом електронском микроскопијом Калцинисани узорци
су синтеровани на 1000 degC са задржавањем од 3 сата у атмосфери ваздуха (CaP1000
CaMgP1000 and CaSrNaP1000) Синтеровани материјали окарактерисани су рендгенском
дифракцијом на поликристалном узорку инфрацрвеном спекторскопијом са
Фуријеовом трансформацијом и скенирајућом електронском микроскопијом са
M Mirković et alScience of Sintering 50 (2018) 95-109 ___________________________________________________________________________
109
енергетско дисперзивном анализом као и методе комплексне импендансе Највећа
проводљивост добијена је на температури од 700 degC и износи 190times10-3
Ω-1
cm-1
за
мешовито допирани фосфатни материјал (CaSrNaP1000) Добијене активационе
енергије проводљивости износе 031 eV у температурном опсегу од 500-700 ordmC
Кључне речи Синтеровање Рендгенска дифракција Електрична проводљивост
Фосфатни материјали Горивне ћелије
copy 2016 Authors Published by the International Institute for the Science of Sintering This
article is an open access article distributed under the terms and conditions of the Creative
Commons mdash Attribution 40 International license
(httpscreativecommonsorglicensesby40)
M Mirković et al Science of Sintering 50 (2018) 95-109
___________________________________________________________________________
106
4 Conclusions
The pure Ca and doped with Mg and NaSr hydroxyapatite (CaP CaMgP and NaSrP)
were successfully synthesized using slightly modified solution precipitation method (partial
substitution nitrates by acetate solutions) and thus we achieved higher solubility and more
cost effective synthesis Any secondary phase that formed during synthesis decomposed
during thermal treatment leaving pure CaP CaMgP and CaSrNaP hexagonal (P63m)
Ca5(PO4)3OH materials at room temperature During sintering process at 1000 ordmC in an air for
3 h we developed high temperature doped calcium phosphate materials ie CaMgP1000 and
CaSrNaP1000 materials SEM analysis showed that CaMgP1000 and CaSrNaP1000 materials have
high structural arrangement EDS analysis confirmed that we obtained phosphate compound
type-M2X2O7 The fine crystalline powders after sintering at 1000 ordmC achieved 98 of
theoretical density The maximum value of electrical conductivity at 700 ordmC was achieved for
CaSrNaP1000 material with amounts to 190times10-3
Ω-1
cm-1
Electrical characteristics of the
sintered samples indicate the superiority of solid solution for the application in SOFCs
Additionally considering high sintering temperature one can conclude that the obtained
phosphate material demonstrates high thermal stability an important characteristics in its
potential use as solid electrolyte However it is important to note that the future research
should include furthering lower temperature synthesis routes improving stability ITndashSOFCs
electrolyteelectrode molecular dynamics (MD) simulations of conduction mechanisms and
the general search for improved ion conductors Also the breadth of possible doping regimes
in this phosphate material provides new opportunities to design and optimize their transport
parameters for potential clean energy applications
Acknowledgements
This work was supported by the Ministry of Education Science and Technological
development of Serbia as part of Projects III 45012 and OI 176016
5 References
1 A V Galvagno Chiodo F Urbani F Freni Int J Hydrogen Energy 38 (2013)
3913
2 Y Takagi K Kerman C Ko S Ramanathan J Power Sources 243 (2013) 1
3 H J Alves C Bley Junior R R Niklevicz E P Frigo M S Frigo C H Coimbra-
Arauacutejo Int J Hydrogen Energy 38 (2013) 5215
4 J-J Liu S-Q Zhang W-D Wang J-F Gao W Liu C-S Chen J Power Sources
287 (2012) 287
5 J L Hertz H L Tuller J Electroceram 13 (2004) 663
6 S J Litzelman J L Hertz W Jung H L Tuller Fuel Cells 8 (2008) 294
7 EY Pikalova A A Murashkina V I Maragou A K Demin V N Strekalovsky
P E Tsiakaras Int Journal Hydrogen Energy 36 (2011) 6175
8 D Magdalena Int J Electrochem Sc 7 (2012) 2874
9 M Stojmenović S Bošković M Žunić B Babić B Matović D Bajuk-
Bogdanović Mat Chem Phys 153 (2015) 422
10 M Stojmenović M Žunić J Gulicovski D Bajuk-Bogdanović I Holclajtner-
Antunović V Dodevski J Mater Sci 50 (2015) 3781
11 A Laghzizil N Elherch A Bouhaouss G Lorente T Coradin J Livage Mater
Res Bull 36 (2001) 953
M Mirković et alScience of Sintering 50 (2018) 95-109 ___________________________________________________________________________
107
12 K Yamashita H Owada T Umegaki T Kanazawa T Futagami Solid State Ionics
28 (1988) 660
13 F Bakan O Laccedilin H Sarac Powder Technol 233 (2013) 295
14 M Sadat-Shojai M-T Khorasani A Jamshidi J Cryst Growth 361 (2012) 73
15 S Pramanik A K Agarwal K N Rai A Garg Ceram Int 33 (2007) 419
16 I Mobasherpour MS Heshajin A Kazemzadeh M Zakeri J Alloys Comp 430
(2007) 330
17 A K Nayak Int Jour ChemTech Res 2 (2010) 903
18 T T Trang Pham T Phuong Nguyen T Nam Pham T Phuong Vu D L Tran
HThai Vietnam Academy of Science and Technology 4 (2013) 1
19 S C Cox P Jamshidi L M Grover K K Mallick Mat Sci Eng C-Bio S 35
(2014) 106
20 E Champion Acta Biomater 9 (2013) 5855
21 S J Kalita A Bhardwaj H A Bhatt Mat Sci Eng C-Bio S 27 (2007) 441
22 S Ramesh K L Aw R Tolouei M Amiriyan C Y Tan M Hamdi Ceram Int
39 (2013) 111
23 Đ Veljović G Vuković I Steins E Palcevskis P S Usković R Petrović Sci
Sinter 45 (2013) 233
24 S Raynaud E Champion D Bernache-Assollant Biomater 23 (2002) 1073
25 Rigaku Corporation Tokyo J PDXL Version 2030 Integrated X-ray Powder
Diffraction Software 2011
26 Powder Diffraction File P-D announcement of new data-base release 2012
International Centrefor Diffraction Data (ICDD)
27 K Momma F Izumi J Appl Crystallogr 44 (2011) 1272
28 M Prekajski M Mirković B Todorović A Matković M Marinović-Cincović J
Luković B Matović J Eur CeramSoc 36 (2016) 1293
29 J Wang LL Shaw Adv Mater 19 (2007) 2364
30 A Mortier J Lemaitre P Rouxet Thermochim Acta 113 (1987) 133
31 R T Downs M Hall-Wallace Am Mineralog 88 (2003) 247
32 G Qi S Zhang K A Khor C Liu X Zeng W Weng Thin Solid Films 516
(2008) 5176
33 S C Cox P Jamshidi L M Grover K K Mallick Mat Sci Eng C-Bio S 35
(2014) 106
34 A Cuccu S Montinaro R Orrugrave G Cao D Bellucci A Sola Ceram Int 41
(2015)725
35 W Zhang Y Shen H Pan K Lin X Liu B W Darvell Acta Biomater 7 (2011)
800
36 J A Gadsden Infrared spectra of minerals and related inorganic compounds
Butterworths London 1975
37 M Iafisco A Ruffini A Adamiano S Sprio A Tampieri Mat Sci Eng C-Bio S
35 (2014) 212
38 S Koutsopoulos J Biomed Mater Res 62 (2002) 600
39 S-C Liou S-Y Chen H-Y Lee J-S Bow Biomaterials 25 (2004) 189
40 M E Fleet Biomaterials 30 (2009) 1473
41 N V Chukanov Infrared spectra of mineral species Extended library Springer
London Limited 2013
42 V Koleva V Stefov M Najdoski A Cahil Thermochim Acta 619 (2015) 20
43 B Mehdikhani G H Borhani J Ceram Process Res 16 (2015) 308
44 P Solanki S Vasant M Joshi Int J Appl Ceram Tech 11 (2014) 663
45 G B Balazs R Glass Solid State Ionics 76 (1995) 155
46 G Buvaneswari K Valsalan Mater Lett 134 (2014) 252
M Mirković et al Science of Sintering 50 (2018) 95-109
___________________________________________________________________________
108
47 A Farzadi F Bakhshi M Solati-Hashjin M Asadi-Eydivand N A Osman Ceram
Int40 (2014) 6021
48 A G Evans C H Hsueh J Am Ceram Soc 69 (1986) 444
49 A J Jacobson Chem Mater 22 (2010) 660
50 A Orera P R Slater Chem Mater 22 (2010) 675
51 L Malavasi C A J Fisher M S Islam Chem Soc Review 39 (2010) 4370
52 D Marrero-Loacutepez M C Martiacuten-Sedentildeo J Pentildea-Martiacutenez J C Ruiz-Morales P
Nuacutentildeez M A G Aranda J Power Sources 195 (2010) 2496
53 M P Moreira G D de Almeida Soares J Dentzer K Anselme L Aacute de Sena A
Kuznetsov Mat Sci Eng C-Bio S 61 (2016) 736
54 F Ren Y Leng R Xin X Ge Acta Biomater 6 (2010) 2787
55 Z Y Li W M Lam C Yang B Xu G X Ni S A Abbah Biomaterials 28
(2007)1452
56 Z Leilei L Hejun L Kezhi F Qiangang Z Yulei L Shoujie Ceram Int 40
(2014) 13123
57 K Y Leung C Calvo Can J Chem 50 (1972) 2519
58 H Larsove J Ingela M Christina Acta Chemica Scandinavica 22 (1968) 1419
59 E Kendrick M S Islam P R Slater J Mater Chem 17 (2007) 3104
60 E Kendrick M S Islam P R Slater Chem Commun 715 (2008) 7
61 K Fukuda T Asaka R Hamaguchi T Suzuki H Oka A Berghout Chem Mater
23 (2011) 5474
62 E Kendrick M S Islam P R Slater Solid State Ionics 177 (2007) 3411
63 L Leon-Reina E R Losilla M Martinez-Lara S Bruque M A G J Mater
Chem 14 (2004) 1142
64 N Nallamuthu I Prakash N Satyanarayana M Venkateswarlu J Alloys and
Compounds 509 (2011) 1138
65 M Sato Y Kono H Ueda K Uematsu K Toda Solid State Ionics 83 (1996)249
66 D Marrero-Loacutepez L dos Santos-Goacutemez L Leoacuten-Reina J Canales-Vaacutezquez E R
Losilla Journal of Power Sources 245 (2014) 107
67 S Nakayama T Kageyama H Aono Y Sadaoka J Mater Chem 5 (1995) 1801
68 Y Masubuchi M Higuchi T Takeda S Kikkawa J Alloys and Compounds 408
(2006) 641
69 J McFarlane S Barth M Swaffer J E H Sansom P R Slater Ionics 8 (2002)
149
70 S Tao J T S Irvine Mater ResBullet 36 (2001) 1245
71 J E H Sansom J R Tolchard P R Slater M S Islam Solid State Ionics 167
(2004) 17
Садржај Метода таложне титрације коришћена је за добијање чистог и допираног
хидроксиапатита јонима Sr Mg и Na (CaP CaMgP and CaSrNaP) За синтезу
супституисаних хидроксиапатитских материјала коришћена је модификована
таложна метода у односу на досада познате методе Због боље растворљивости и
знатно веће исплативости коришћени су раствори ацетата уместо нитрата
Синтетисани материјали су калцинисани на 400 degC а затим окарактерисани
рендгенском дифракцијом инфра црвеном спектроскопијом са Фуријеовом
трансформацијом и скенирајућом електронском микроскопијом Калцинисани узорци
су синтеровани на 1000 degC са задржавањем од 3 сата у атмосфери ваздуха (CaP1000
CaMgP1000 and CaSrNaP1000) Синтеровани материјали окарактерисани су рендгенском
дифракцијом на поликристалном узорку инфрацрвеном спекторскопијом са
Фуријеовом трансформацијом и скенирајућом електронском микроскопијом са
M Mirković et alScience of Sintering 50 (2018) 95-109 ___________________________________________________________________________
109
енергетско дисперзивном анализом као и методе комплексне импендансе Највећа
проводљивост добијена је на температури од 700 degC и износи 190times10-3
Ω-1
cm-1
за
мешовито допирани фосфатни материјал (CaSrNaP1000) Добијене активационе
енергије проводљивости износе 031 eV у температурном опсегу од 500-700 ordmC
Кључне речи Синтеровање Рендгенска дифракција Електрична проводљивост
Фосфатни материјали Горивне ћелије
copy 2016 Authors Published by the International Institute for the Science of Sintering This
article is an open access article distributed under the terms and conditions of the Creative
Commons mdash Attribution 40 International license
(httpscreativecommonsorglicensesby40)
M Mirković et alScience of Sintering 50 (2018) 95-109 ___________________________________________________________________________
107
12 K Yamashita H Owada T Umegaki T Kanazawa T Futagami Solid State Ionics
28 (1988) 660
13 F Bakan O Laccedilin H Sarac Powder Technol 233 (2013) 295
14 M Sadat-Shojai M-T Khorasani A Jamshidi J Cryst Growth 361 (2012) 73
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47 A Farzadi F Bakhshi M Solati-Hashjin M Asadi-Eydivand N A Osman Ceram
Int40 (2014) 6021
48 A G Evans C H Hsueh J Am Ceram Soc 69 (1986) 444
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52 D Marrero-Loacutepez M C Martiacuten-Sedentildeo J Pentildea-Martiacutenez J C Ruiz-Morales P
Nuacutentildeez M A G Aranda J Power Sources 195 (2010) 2496
53 M P Moreira G D de Almeida Soares J Dentzer K Anselme L Aacute de Sena A
Kuznetsov Mat Sci Eng C-Bio S 61 (2016) 736
54 F Ren Y Leng R Xin X Ge Acta Biomater 6 (2010) 2787
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(2007)1452
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61 K Fukuda T Asaka R Hamaguchi T Suzuki H Oka A Berghout Chem Mater
23 (2011) 5474
62 E Kendrick M S Islam P R Slater Solid State Ionics 177 (2007) 3411
63 L Leon-Reina E R Losilla M Martinez-Lara S Bruque M A G J Mater
Chem 14 (2004) 1142
64 N Nallamuthu I Prakash N Satyanarayana M Venkateswarlu J Alloys and
Compounds 509 (2011) 1138
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Losilla Journal of Power Sources 245 (2014) 107
67 S Nakayama T Kageyama H Aono Y Sadaoka J Mater Chem 5 (1995) 1801
68 Y Masubuchi M Higuchi T Takeda S Kikkawa J Alloys and Compounds 408
(2006) 641
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149
70 S Tao J T S Irvine Mater ResBullet 36 (2001) 1245
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(2004) 17
Садржај Метода таложне титрације коришћена је за добијање чистог и допираног
хидроксиапатита јонима Sr Mg и Na (CaP CaMgP and CaSrNaP) За синтезу
супституисаних хидроксиапатитских материјала коришћена је модификована
таложна метода у односу на досада познате методе Због боље растворљивости и
знатно веће исплативости коришћени су раствори ацетата уместо нитрата
Синтетисани материјали су калцинисани на 400 degC а затим окарактерисани
рендгенском дифракцијом инфра црвеном спектроскопијом са Фуријеовом
трансформацијом и скенирајућом електронском микроскопијом Калцинисани узорци
су синтеровани на 1000 degC са задржавањем од 3 сата у атмосфери ваздуха (CaP1000
CaMgP1000 and CaSrNaP1000) Синтеровани материјали окарактерисани су рендгенском
дифракцијом на поликристалном узорку инфрацрвеном спекторскопијом са
Фуријеовом трансформацијом и скенирајућом електронском микроскопијом са
M Mirković et alScience of Sintering 50 (2018) 95-109 ___________________________________________________________________________
109
енергетско дисперзивном анализом као и методе комплексне импендансе Највећа
проводљивост добијена је на температури од 700 degC и износи 190times10-3
Ω-1
cm-1
за
мешовито допирани фосфатни материјал (CaSrNaP1000) Добијене активационе
енергије проводљивости износе 031 eV у температурном опсегу од 500-700 ordmC
Кључне речи Синтеровање Рендгенска дифракција Електрична проводљивост
Фосфатни материјали Горивне ћелије
copy 2016 Authors Published by the International Institute for the Science of Sintering This
article is an open access article distributed under the terms and conditions of the Creative
Commons mdash Attribution 40 International license
(httpscreativecommonsorglicensesby40)
M Mirković et al Science of Sintering 50 (2018) 95-109
___________________________________________________________________________
108
47 A Farzadi F Bakhshi M Solati-Hashjin M Asadi-Eydivand N A Osman Ceram
Int40 (2014) 6021
48 A G Evans C H Hsueh J Am Ceram Soc 69 (1986) 444
49 A J Jacobson Chem Mater 22 (2010) 660
50 A Orera P R Slater Chem Mater 22 (2010) 675
51 L Malavasi C A J Fisher M S Islam Chem Soc Review 39 (2010) 4370
52 D Marrero-Loacutepez M C Martiacuten-Sedentildeo J Pentildea-Martiacutenez J C Ruiz-Morales P
Nuacutentildeez M A G Aranda J Power Sources 195 (2010) 2496
53 M P Moreira G D de Almeida Soares J Dentzer K Anselme L Aacute de Sena A
Kuznetsov Mat Sci Eng C-Bio S 61 (2016) 736
54 F Ren Y Leng R Xin X Ge Acta Biomater 6 (2010) 2787
55 Z Y Li W M Lam C Yang B Xu G X Ni S A Abbah Biomaterials 28
(2007)1452
56 Z Leilei L Hejun L Kezhi F Qiangang Z Yulei L Shoujie Ceram Int 40
(2014) 13123
57 K Y Leung C Calvo Can J Chem 50 (1972) 2519
58 H Larsove J Ingela M Christina Acta Chemica Scandinavica 22 (1968) 1419
59 E Kendrick M S Islam P R Slater J Mater Chem 17 (2007) 3104
60 E Kendrick M S Islam P R Slater Chem Commun 715 (2008) 7
61 K Fukuda T Asaka R Hamaguchi T Suzuki H Oka A Berghout Chem Mater
23 (2011) 5474
62 E Kendrick M S Islam P R Slater Solid State Ionics 177 (2007) 3411
63 L Leon-Reina E R Losilla M Martinez-Lara S Bruque M A G J Mater
Chem 14 (2004) 1142
64 N Nallamuthu I Prakash N Satyanarayana M Venkateswarlu J Alloys and
Compounds 509 (2011) 1138
65 M Sato Y Kono H Ueda K Uematsu K Toda Solid State Ionics 83 (1996)249
66 D Marrero-Loacutepez L dos Santos-Goacutemez L Leoacuten-Reina J Canales-Vaacutezquez E R
Losilla Journal of Power Sources 245 (2014) 107
67 S Nakayama T Kageyama H Aono Y Sadaoka J Mater Chem 5 (1995) 1801
68 Y Masubuchi M Higuchi T Takeda S Kikkawa J Alloys and Compounds 408
(2006) 641
69 J McFarlane S Barth M Swaffer J E H Sansom P R Slater Ionics 8 (2002)
149
70 S Tao J T S Irvine Mater ResBullet 36 (2001) 1245
71 J E H Sansom J R Tolchard P R Slater M S Islam Solid State Ionics 167
(2004) 17
Садржај Метода таложне титрације коришћена је за добијање чистог и допираног
хидроксиапатита јонима Sr Mg и Na (CaP CaMgP and CaSrNaP) За синтезу
супституисаних хидроксиапатитских материјала коришћена је модификована
таложна метода у односу на досада познате методе Због боље растворљивости и
знатно веће исплативости коришћени су раствори ацетата уместо нитрата
Синтетисани материјали су калцинисани на 400 degC а затим окарактерисани
рендгенском дифракцијом инфра црвеном спектроскопијом са Фуријеовом
трансформацијом и скенирајућом електронском микроскопијом Калцинисани узорци
су синтеровани на 1000 degC са задржавањем од 3 сата у атмосфери ваздуха (CaP1000
CaMgP1000 and CaSrNaP1000) Синтеровани материјали окарактерисани су рендгенском
дифракцијом на поликристалном узорку инфрацрвеном спекторскопијом са
Фуријеовом трансформацијом и скенирајућом електронском микроскопијом са
M Mirković et alScience of Sintering 50 (2018) 95-109 ___________________________________________________________________________
109
енергетско дисперзивном анализом као и методе комплексне импендансе Највећа
проводљивост добијена је на температури од 700 degC и износи 190times10-3
Ω-1
cm-1
за
мешовито допирани фосфатни материјал (CaSrNaP1000) Добијене активационе
енергије проводљивости износе 031 eV у температурном опсегу од 500-700 ordmC
Кључне речи Синтеровање Рендгенска дифракција Електрична проводљивост
Фосфатни материјали Горивне ћелије
copy 2016 Authors Published by the International Institute for the Science of Sintering This
article is an open access article distributed under the terms and conditions of the Creative
Commons mdash Attribution 40 International license
(httpscreativecommonsorglicensesby40)
M Mirković et alScience of Sintering 50 (2018) 95-109 ___________________________________________________________________________
109
енергетско дисперзивном анализом као и методе комплексне импендансе Највећа
проводљивост добијена је на температури од 700 degC и износи 190times10-3
Ω-1
cm-1
за
мешовито допирани фосфатни материјал (CaSrNaP1000) Добијене активационе
енергије проводљивости износе 031 eV у температурном опсегу од 500-700 ordmC
Кључне речи Синтеровање Рендгенска дифракција Електрична проводљивост
Фосфатни материјали Горивне ћелије
copy 2016 Authors Published by the International Institute for the Science of Sintering This
article is an open access article distributed under the terms and conditions of the Creative
Commons mdash Attribution 40 International license
(httpscreativecommonsorglicensesby40)